I don't think I'd be comfortable with less than four Crucibles (or conceivably three). I think the reasons have been chewed on often enough, but just for the hell of it:
I can't think of many non-combo decks that aren't susceptible to Waste/Crucible. If that was the only thing, okay. But it also helps alleviate the big beater problem with Factories (incidentally protecting one of the very few real win conditions), and offsets the disadvantage inherent in the mana base (Diamonds, Cities, and - indirectly - Tombs; which makes the early game more stable as well as faster). Smokestack is kind of slow. Well, with Crucible it's faster. (Plus, it gets rid of "dead" Crucibles). I can now play Armageddon even if I'm behind instead of just when I'm on the offensive (which makes taxing better). It makes my Factories attack into Bobs.

A first- or second-turn Crucible does not do nothing. It may do nothing now, but if it'll do impressive stuff a little later, that's fine. It's not like I'm going for the tenth-turn win.