arl: To stay on track, perhaps someone could correct me [I presume that what you mean here is “convince”, as the proposition you describe seems to be diametrically opposed to what you seemed to be supporting in earlier posts] in thinking that if we give more people less money rather than less people more money that we actually, investment-wise, come to the same point in less steps.
Why, jshore gave many of the arguments for this in his very first post in this thread and in several subsequent ones. To summarize (and I hope jshore will correct me if I misstate his views): Start from the premise that the “point” we want to get to is increased prosperity and economic opportunity for society as a whole. Accept the hypothesis that the instrument we’re going to use to try to achieve this is a tax cut eliminating X billion dollars from the revenue stream. The question: how do we distribute this tax cut so that it most effectively influences the workings of the economy towards this goal?
Classic “trickle-down” or “supply-side” theory (which is what you seem to be arguing for) says that you should direct most of the X billion of lost revenue to the comparatively few people who pay large amounts of tax (this is the effect that an “across-the-board” tax reduction will have). These people, most of whom already have plenty of money, don’t need the extra revenue for their personal needs so they will use it as an investment tool to make more money by creating new businesses and jobs and so forth, thereby stimulating the economy and making everybody better off. Ta-da.
And it sounds great. But what jshore and others have pointed out is that “trickle-down” requires a pre-existing healthy demand side of the economy in order to have the desired effect. If most of the non-wealthy are struggling financially, they are not going to be able to provide the consumer demand that gives supply-side investors an acceptable return on their investment. (Plus they are apt to have a host of related problems—higher crime, lower education, etc.—that makes them less attractive as participants in these new enterprises too.) So the investors, being no dummies, will move the bulk of their new wealth to places where it can produce a profitable return, and that vanished revenue ends up stimulating the US economy absolutely not at all. Bzzzzt: failed plan!
Of course, the wealthy investors will still pump up our economy somewhat by increasing their consumption (assuming they don’t trend overwhelmingly towards foreign luxury consumables that also provide little or no benefit to our economy), but not enough to justify the public investment in a major revenue-losing tax cut for them. (There are also disadvantages associated with the fact that boosting supply-side investment can ultimately be expensive for the public in terms of requiring more regulation and oversight. That is, when wealthy people suddenly have a whole lot more money to put into profit-making ventures, many of them will be smart enough to notice that there’s a sh*tload of money to be made in enterprises that degrade the environment or exploit the consumer—and some of them, alas, will not be conscientious enough to resist such temptations.)
“Trickle-up”, on the other hand, relies on the fact that the non-wealthy tend to be more constrained by necessity in how they use their money. It says that you should direct the tax cut primarily to the large numbers of people who pay comparatively little tax (which includes most of the middle class). They will spend it predominantly locally on consumables (thus getting more money back into the pockets of the supply-side producers of the consumables) and such “social capital investments” as health care and education (thus improving the future prospects for a qualified and productive workforce).
And because there are simply so many more of the non-wealthy than the wealthy, their increased consumption will provide a much bigger economic boost than increased consumption by the wealthy, as well as being more reliable than the unfettered, globalized, completely discretionary investment decisions of the wealthy. (I have wondered, actually, whether this prospect is affected significantly by the size of the trade deficit: that is, if non-wealthy Americans use their tax-cut money primarily to buy foreign-made goods, is it still a boost for our economy? Presumably the fact that it still feeds money into domestic marketing and distribution networks is a plus, but I’d be pleased to hear some “trickle-upper” go into the question in more detail.)
Okay then: should we trickle up or down? It seems clear to me that that choice depends on how the supply and demand sides of the economy are doing at the time you ask the question. In a society where poverty is low, education levels and general quality of life are high, unemployment is persistently high, and investment capital is in relatively short supply, it makes a lot of sense to me that you would prefer to stimulate the economy by trying to inject more money into the supply side. Get some of that juice flowing and try to tempt investors into shooting for the big bux. This seems to be one of the approaches that some European countries are now taking (some very cautiously, some (like the UK) less so) to their problems of sluggish economies and high unemployment.
For us in the US, on the other hand, it looks pretty obvious that the demand side is currently much more at risk than the supply side. Upward redistribution of wealth has been occurring at a tremendous pace over the past couple of decades, while incomes, benefits, and social capital for the poor and working-class have improved hardly at all in real terms over the same period. Consumer debt is really big, average household savings are tiny, homelessness is rising again, infrastructure’s deteriorating—eek! The supply side these days is absolutely stinky with money to invest (much of which is going overseas), while the economic and political power of the demand side continues to erode. Supply-side prosperity obviously won’t always guarantee the health of the demand side, so we’d better apply some economic stimulus to the demand side directly before it loses its ability to support the supply-side investments that we want to encourage.
So you see, arl, the idea isn’t necessarily to get to “the same point in [fewer] steps”, it’s to get to the same point in more effective steps. There’s simply no point in giving American investors more money to invest if they won’t find America an attractive place to invest it, or if they can do lots better elsewhere (as in Mexico or the Philippines). The argument behind “trickle-up” is that we’ll get better results for the economy as a whole out of the same X billion dollars by giving it to those who are likely to use it in a more directly stimulating way. There, how’s that? Are you convinced yet? 
It seems to me that distributing money this way to stimulate the economy is, at best, planting seeds by throwing them up in the air and letting the wind guide it.
How you figure? It’s certainly no more random in its effects than giving the money to the supply-side investors: in fact, as I’ve been pointing out, giving money to investors is less reliable, since they are more likely just to take their “seeds” somewhere else where you get no use out of the crop.
Which is, you’ll pardon me saying, the opposite of our goal if we want to control the economy for more efficient growth. Or do I not understand the goal correctly?
Yes, I think you’ve misunderstood the goal somewhat. We’re not really talking about “controlling” the economy: flowbark gave an excellent explanation of why such command/control economies as the Soviet one don’t do a very good job of promoting economic growth. We want to influence the economy by providing incentives that take advantage of the ways markets naturally work (e.g., when people get extra money they tend to use it, wealthy people tend to invest it, non-wealthy people tend to spend it) to encourage the desired outcomes.
And the whole point of doing this via a tax cut is to take advantage of people’s willingness to use their individual preferences and judgements in following these incentives, rather than requiring expensive micromanaging controls on the details of the expenditures. (Imagine if every citizen who got a tax cut demanded to be reimbursed for his/her efforts in deciding how to use the money, because it acts as a helpful stimulus to the economy—or demanded that an OMB official direct his/her expenditures to save him/her the burden of making the decision! We’d swallow up whatever economic growth we produced in short order. See, even we hated liberals do recognize the practical advantages of freedom 
—we don’t argue that everything should be controlled by the bureaucracy! We just recognize as well the practical advantages of checks and balances on freedom, which include government provision of particular economic incentives.)
Besides which, it seems to be fairly commonly stated (although I must admit that I don’t know what the arguments for this statement are) that the economic boom of the 90s was largely demand-driven. Notice how panicky everyone is getting at signs that consumer demand is slackening!