I'm fairly certain that's even in the book. A large number of pilots reject it because then after you park and shutdown you can smell a little exhaust getting sucked into the APU inlet.

I think that's somewhat silly since we're getting exhaust from other aircraft as we taxi around but it seems to be the primary objection. It's also critical to distinguish between fume events (your own burning, atomized oil) and just stinky exhaust. One is a major health hazard and the other is just an olfactory annoyance.