Without the Establishment Clause, there is no separation of church and state. Assuming the other parts of the First Amendment are left intact, the state would not be able to make it illegal to practice any religion or to express any religious opinion, but the state could establish a particular denomination as the Official Church, and tax all citizens to pay for it. Of course, the state would still be subject to the check of democratic elections, and it’s not likely the voters would vote for that–then again, that argument could be used for all of the guarantees in the Bill of Rights, couldn’t it? (“Oh, The People would never vote for censorship”–or “witch burnings” or “having people shot without trial”.) The state could do all sorts of things short of establishing an official Church of America–it could register all citizens as members of whatever church they were born into, and finance those churches via payroll deducations, just like Social Security or the income tax, unless citizens specifically demanded to be opted out of the system. I believe Germany has or had a system somewhat like this.
Supporters of SOCAS have a strong suspicion–reinforced by much history–that separation of church and state is the best guarantee of religious liberty, and that reducing or eliminating that separation puts religious freedom (and other freedoms, like freedom of speech) in danger.
Of course Germany is hardly a theocratic hellhole. Neither is England, which still has a full-fledged state church. But the coexistence of state churches in England (and some other European nations) with religious freedom and general personal liberty is partly a happy accident of history. It wasn’t always that way–they did used to kill and torture each other over religio-political issues in England. Even as late as the 19th Century, Catholics, Protestant “dissenters”, and atheists were barred from being in Parliament, attending Oxford and Cambridge, etc. The Established Church in England is a vestigial church, a relic of the past. (It may not last much longer either, as it’s becoming increasinly demographically irrelevant, and England is becoming more religiously diverse.) A Church of America might eventually become as benign–after a thousand years of civil strife. And Germany isn’t the U.S.–the U.S. is both fervently religious and mind-boggling diverse in its fervency. Many European countries are more or less mono-sectarian or oligo-sectarian–everyone is Catholic or Lutheran or Calvinist (or at least traditionally so–in many Western European countries these allegiences are pretty nominal), or else the population is divided between two or three groups (Catholic and one or two Protestant traditions side by side, but not the fantastic array of sects, denominations, and independent churches we have here). Opening up that can of worms here would be a huge mess–pork barrel politics and American sectarianism–yikes!
SOCAS is also just the right thing to do–it’s wrong to make atheists pay for the Federal Bureau of Missions and Evangelism (or anything tantamount to that). It’s wrong to make Christians pay for the State Ministry of Atheist Propaganda. This is so even if atheists and Christians aren’t actually thrown in jail for their opinions (unless maybe it’s for tax evasion when they refuse to pay for the Federal Bureau of Missoins and Evangelism or the State Ministry of Atheist Propaganda). People shouldn’t be second-class citizens in their own country on account of their religious opinions.
Finally, a couple of quotes from one of our Founding Fathers (cue patriotic music and picture of waving Star-Spangled Banner):
“Who does not see that the same authority, which can establish Christianity in exclusion of all other religions, may establish, with the same ease, any particular sect of Christians, in exclusion of all other sects; that the same authority, which can force a citizen to contribute threepence only of his property, for the support of any one establishment, may force him to conform to any other establishment, in all cases whatsoever?” – James Madison, the Memorial and Remonstrance (that link, incidentally, is to the homepage of a church.)
“…it is proper to take alarm at the first experiment on our liberties. We hold this prudent jealousy to be the first duty of Citizens, and one of the noblest characteristics of the late Revolution.” – James Madison again (actually, this quote is also from the Memorial and Remonstrance–but this link goes to a bunch of atheists…)