I’ll bite - Congress should have the power to attach riders, even when they are (arguably) irrelevant to the purpose of the bill, and would be unlikely to pass on their own merits. This is because:
1.) Who decides what’s relevant? McCain’s amendment to ban torture was attached to a defense funding bill - there’s a strong argument to be made that it wasn’t at all relevant. The funding bill was intended to support policy, not to steer it (in theory, anyway). McCain’s amendment was a dramatic policy change, not at all in keeping with the overall purpose of the bill. This brings me to my next point,
2.) Sometimes, we need irrelevant riders. On issues where lobbying groups (or the White House) would otherwise be able to bring enough pressure to bear to kill a stand-alone measure, attaching it to a bill which must pass can give it a fighting chance. As in the case of McCain’s amendment, this can be a remarkably good thing. It isn’t so clearly the case with the gambling bill - but if you believe that gambling is a serious and crippling social problem, then this rider is a godsend. It wouldn’t have passed on its own, but now that it has, we’re one step closer to solving this problem.
Now, personally, I strongly disagree with this legislation, and I’m not at all convinced that online gambling is even a problem that has to be solved. But this brings me to point three:
3.) The system can correct itself. There are a lot of very smart, very attentive people watching everything congress does. This online gambling measure was introduced at the last minute, but it wasn’t “sneaked” in - legislation doesn’t sneak. There are a lot of people who oppose it, and they’re speaking out very loudly about it. Now it’s in the public’s court. If this legislation is so contrary to the popular will that voters get outraged, Congress will feel the heat and repeal it. On the other hand, the voters might actually think this is a decent policy - in which case, no harm, no foul.
Caveat: There is, of course, the problem of riders which are only mildly unpleasant - that is, no one outside a small interest group actually wants them, but they don’t hurt the rest of us, and so they slip through. I would argue, however, that this is a fair price to pay for the legislature’s ability to quickly pass controversial legislation while shielded from special interests.
(Additional caveat: I’m not sure I actually believe any of this - but you wanted a debate, a debate you’ve got.)