I am one of those officers who prefers the Class A uniform. We have some "relaxed" options at my department, but I choose to wear the long sleeve shirt with tie and under vest in cold weather. I find I carry myself with more confidence when I look very sharp. People notice.
As for "functionality," I sometimes think officers hope that a relaxed uniform will make up for being in poor shape. In my shirt and tie, I was able to scale a 10 foot fence and climb down a sign on the interstate to pull down a jumper. In my high gloss oxfords I was able to run down a guy in a foot pursuit. I've never felt that this uniform has kept me from work. If anything, I'd think cargo pants full of "stuff" with a bunch of Molle gear on a tac vest would create more problems than a tie.
I do agree that the vest under the shirt gets really freaking hot in the summer. But the external carrier gets hot too... The only benefit is that you can take it off at the station. I see too many guys taking it off in the car, at lunch, etc., which I don't agree with. Never in public.