bryceman - i guess theoretically the idea is that two clamping points is better than one. imagine trying to hold a stick horizontal with one hand in the middle vs. with two hands - much easier with two hands, since it is secured at two points rather than one, so there's really no way for it to rotate. but i bet there's really no significant difference with stems and handlebars because the distance between the bolts is so small relative to the bar length.

the only other concern i can think of would be that with carbon bars, 4 bolts might distribute clamping force over a larger area, making damage to the bar less likely...but again, there's probably no real significant difference.