As we all know, airline pilots wear uniforms to make them readily identifable and impressive. If there is an emergency and the pilots or cabin crew members have to give orders to the pax, you don't want any confusion or resistance ("who is that guy in the golf shirt to tell ME not to evacuate on the side that's on fire?").
Such considerations don't apply to most instructional situations. Training flights generally are manned by one instructor and one or at most two students. Everybody knows perfectly well who is in command, who has the most experience, and who has the final say over how the flight is conducted. The instructor doesn't need a fancy uniform to set himself or herself apart from the other occupant(s) of the aircraft. And of course, Mister Geezer's point is valid.
I certainly agree that flight instructors meet most (though not all) of the criteria for being "professionals" ... but that status is not dependant upon fancy dress. A doctor without a white lab jacket, or an ungowned barrister, is still a professional.
Finally, at the risk of appearing to contradict myself, if someone (pilot or not) wants to wear wings on their chest or bars on their shoulders, IMHO that's their choice and their business. Pretentious? Sure, but they're not hurting anyone, so big deal. Let them have their fun (so long as it doesn't go to their heads and result in them bossing everyone else around, of course).