It had occurred to me, as I'm sure it occurred to many, that 33 round pistol magazines aren't used in hunting, and probably not in home defense, either, begging the question of precisely what the usefulness of such a magazine is. But I know the general stance of the Second Amendment caucus is that no additional regulation shall be passed, because, even if a regulation is more cosmetic than serious, it sets a bad precedent, re-affirms the government's right to impose other regulations, and ultimately moves the possibility of a real "gun grab" slightly along the confiscatory track.
Still, I'm always less than impressed by that sort of slippery slope argument. There is some truth in the slippery slope argument, but generally the argument is put forward when there are few other good arguments available -- thus the argument becomes not that x is so catastrophic, but x makes y more likely, and y is bad, and y makes z more likely, and z is terrible. But I can't avoid the implication contained in this argument -- So you're saying x, by its own terms, really isn't all that bad? Except to the extent that it makes y possible? Well, can't we just stop y, then?
A couple questions for Ace: Why just pistols? Surely rifles with a 30 round capacity "aren't used in hunting, and probably not in home defense"?
Also why cap at 30 rounds? Why not 10?
As for your dismissal of the slippery slope, see the previous questions. If a law passes that just limits pistol magazines to X then it becomes far easier to limit all magazines to Y
Also contrary to what you say, the slippery slope is exactly how England's gun bans were put into place. Someone goes on a killing spree and the politicians "do something". And then a few years later they do something again. Sound familiar?
The slippery slope / ratchet effect is also exactly what the gun banners over here want. Why accommodate them for no real effect?
Given Ace later says: "[N]or do I think it will prove to be terribly effective at all; it's mostly (mostly) symbolic pap that can only have the most trivial effect on things either way."
And this touches on effectiveness and enforceability. Such a law would really do nothing but hassle the law abiding. Why cede this ground to the gun banners? It has no upside to the 2nd Amendment folks, and a great very likely downside.