Although some classier FBOs definitely cater to corporate crews, I don't think that a corporate crew is any more entitled to a car than a 172 renter pilot. Anybody can be on a tight schedule, and like Lloyd said, everybody deserves to eat.
When I was flying aerial photography, if the WX was good we would fly about 4 hrs in the morning, land and do a quick lunch, then back up for 4 more hours (then maybe a third afternoon flight during long days). I can't remember how many times I was short on time and very hungry at some FBO when the person behind the desk said "Oh, the King Air/Citation/Lear crew has the car right now...they've had it for a couple hours now...should have had it back by now...I guess you can wait if you want, they 'should' be back any time...." I would imagine that busy CFIs don't always have the time to hop around to other airports to find a car, either.
In my opinion, everybody deserves to use the car within the specified time limits. On the flip side, anybody (corporate and charter crews, CFIs, average joe renter pilot, etc.) can potentially abuse the privilege, piss off everybody else, and make themselves look bad when they keep the car too long.