Actually, the flight suit does have its place.
I've recently started instructing again in a small two-place heli. The only stowage is under the seats. (No access when seated!) I have found that my specs/sunspecs/mobile phone get in the way when they are in my pockets and my flight board and maps are serious lose article potential.
Said articles could all be stowed in leg pocket of suit, except flightboard, for which substitute small kneeboard, or use flight suit kneepad.
So I'm seriously considering getting out one of my old suits....worn in both military and civil employ....but minus the regalia if its just instructing work.
And BTW, they are Nomex, so offer added protection.
Agree with many of the other comments. Dress smart:act smart. But smart doesn't need to be formal.