Simply putting the suit on by the aeroplane is a good shout.
The problem I see with that, is that if perchance you are stopped while driving to/from the airport, how are you going to explain your lack of trousers? Seriously, while taking trousers on/off in the car is certainly possible, I'm not sure I see the point.
I think this must be a UK/US thing. Many (though not all) of my instructors wear flight suits. As Genghis says, they're extremely practical, they keep grease and much off your normal clothes, you can leave pens, torches/flashlights, emergency escape hammers, ... in the pockets. Maybe in the UK people worry more about looking silly, "what will the neighbours say?" Here nobody gives a r@t$ar$3.
As it happens, right now I'm off to fly the Citabria, mainly for tailwheel currency but I'll probably also do some light acro. I'll fly over there in my 182. And I'll drive to the airport in my car. On the way home, I need to stop at the drugstore. Why would I do anything other than put on my flightsuit at home, in comfort, and wear it til I get back? I'm not trying to impress anyone, just keep my life simple.
Incidentally if you want another reason/excuse for wearing a flight suit, I noticed that they sell them at Mr X, San Francisco's premier fetish gear shop. So you can always pretend you're on your way to a fetish party (just saying...).