I did my PPL training in LEJR; next to a very big and famous ATO with integrated programs.
Always found it funny to see all the students dressed up in their uniforms looking down at me for flying in my t-shirt, shorts and sneakers. Then again, they are pretty much only allowed to fly 2 hour local flights whereas I have flown almost everywhere the range of a C172 will take me, including Morocco (I must be one of the few fortunate PPLs whose aircraft renter allowed him to fly as he pleases, everybody I asks tells me about idiotic restrictions the renters impose). Then the authorities reduce the hours of an integrated course to 150 because in their eyes these hours are of "better quality"...
Sorry for the rant. A "simple" PPL flying his friends to lunch in a C172 wearing the uniform by own will, not having to, strikes me as pretentious. But if they are happy like that, by all means...
Regarding the flying to third world countries, it might be a good idea. I have flown to Morocco in my swimming suit and had no troubles whatsoever. But I imagine less developed countries could find it odd. I agree this is situation in which it might be a good idea to wear a uniform. Not within the EU though...