well true a belt can weaken muscles in the back and abs but not using them at all is not the greatest choice either.
quick decision????
your going to do bicep curls do you need to wear a belt?..... HECK NO!!!!!!!!!
that is what creates weak muscles. if you cant curl without a belt you are seriously making yourself weaker. (injuries are the exception.)
on deadlifts.........if you dont need it then why use it. if i go under 2-3 reps i will put on a belt, or if i try some outragously heavy set. but i never have one on during warmups.
the bottom line imo is this. if you dont need it dont use it. if its going to be a hard set dont use the belt. if its close to a max or a max go ahead, there is no since holding back big numbers due to not using the belt. but that doest mean you cant attempt a max belt free as well.
I use to have a pretty bad nagging back injury. I am pretty confident i got the injury portauging(sp) canoes from anywhere from 1 to 5 k.. After I stopped using the belt, my back is amost 100%.