If it were up to me, they all would run stock Froyo, Gingerbread, Ice Cream Sandwich, whatever...I can’t think of a single carrier specific app that I ever use. I don’t really use the manufacturer skins or launcher either. I like my phone to be the way ***I*** want it. That’s what’s nice about the Nexus phones...no bloat, just stock Android with updates as soon as Google pushes them out the door.
The skins are of varying usefulness. Those not inclined to tinker probably like them more. You can get most of the usefulness from other apps if you so choose. I believe it is Acer who allows you to choose to use their skin on bootup which I think is genius and should be a requirement for all manufacturers.
As for the carrier stuff, I guess the one benefit is that it could help lower the cost of the phone. I’m still crossing my fingers that T-Mobile gets a pretty standard SGS II, but $800 to get it off contract is a pretty high price. I can deal with some bloat to bring that down to $500 or so.