Well, that's quite easy:
Dual Wielding gives you a boost on available ammo, but therefore you sacrifice accuracy.
In the end it should increase your damage in close quarters, but reduce it over range.
They reappeared in Rainbow Six Vegas 2 and Ghost Recon...but to be honest, Vegas 2 < Vegas 1 and I feel like Advanced Warfighter 2 didn't get the attention it deserved.
I liked the R6V2 multiplayer better than the one of the first. However, that could be mainly for the 16:9 support of Vegas 2 only. In fact, I still play R6V2 in multiplayer from time to time.
GRAW never did pull me in like Vegas, and to be truthful I always wanted to play online but decided to be frugal and only did the demo. Always wanted my friends to switch from Halo to Vegas though.
Well, Vegas is already really slow compared to modern shooters. But GRAW is even slower. You run, you're dead. You sprint, you're dead twice. You crouch, you're most likely dead in a second. And it really takes a lot of time to get into the GRAW multiplayer, because it's so damn hard to spot someone at all.