I suppose recruiters and management from the likes of FlyBE, Thomson Fly, Thomas Cook, XL, NetJets, British Airways, EasyJet, etc. would be so impressed by an FTO full of individuals wearing whatever they like that they would gladly take their sponsoring/mentoring/tagging schemes to such FTOs. NOT!
I suspect their choice of where to run their programmes might have something not only to do with the quality of training, the professionalism of the FTO's staff and management, as well as it's facilities, but also the decorum and professionalism of it's current students. The best way for them to tell what they will get if they choose an FTO for a cadet programme is to look at who is already training there.
This argument won't be persausive to the naysayers here, as they've already got theirs, but it is one of a few legitimate reasons an FTO might require uniforms. Airline recruiters walk the halls of the schools being slagged here with some frequency, so whether you have a choice what to wear or not, make sure it is smart, as you never know who you will bump into or how good their memory will be at a later interview if they see you looking less than professional around your FTO now.