The 'bad part' to the passenger is that he may end up in an aircraft other than the one he opted for...
I'm guessing that the majority people book the flight and not the aircraft type. I've yet to witness anyone stomping off at check in after being told "the aircraft is a 747 today, not a 777" However, I have seen some have a huge hissy fit that their pre booked seat has gone west!!
The main thing is you arrive at your destination safely?