Much like Borderlands you have good accuracy while hip firing, however on an assault rifle you get lots of recoil, and on an SMG you get a bit less, with an LMG you get an ungodly amount. now what the stated games do, is when you aim down sights, its more to make precise shots at a long range, as well as lower the insane amounts of recoil.
Most players will play with single shot on assault rifles when at a long range, and at a closer range, bring a burst fire/full auto SMG.
now Rainbow Six Vegas, isn't exactly a twitch shooter, i'm aware of that. But the only guy who was actually classified as doing a long-range engagement is that sniper WAAAAAY in the back of the map, picking off those guys behind cover with the teeny bit of their head poking out.
Now from a professional standpoint (I've played Rainbow Six Vegas 2 professionally for awhile, before I realized it wasn't fun anymore competitive like that.) Hip firing should do pretty well at a CQB or Close - Mid range engagement, but once you get to shooting at a guy thats about as big as your pinky on screen, you need to be able to aim a bit.
(Yes, I was that guy WAAAAAAAAAY in the back of the map, getting those guys that thought they were fine behind their cover.)