I am against them too. For one thing, I want my kids to be individuals with the ability to dress themselves within rules (certain hemline lengths, etc.) but still be true to their own choices. I expect someday they'll work for an employer like mine, who says employees must buy their own clothes but they must fit these parameters. If they work for a place that requires uniforms, then coolio, but at least they'll have some practice picking and wearing appropriate clothing day after day, mixing and matching in different combos, and (eventually--high school) budgeting their clothing purchase.
For another, the uniforms sold in my area--private school only--are wildly expensive for my income level. Two uniforms cost the equivalent of what I spend at yard sales/consignment for a week of clothing. I expect my kids will want something nicer as they get older, but for now if I can buy something that can be worn for multiple uses for less than a one-use-only set of clothing, I certainly will. I also would prefer to not have to wash two uniforms twice a week and have to iron them, etc. as well.