I never carry a folder in my back pockets. I have tried it once and damaged car seats, couch cushions, and pocket clips that way. I have also almost lost a knife that way as well. On top of that it is terribly uncomfortable. Just my opinion though.
Perhaps I've got a narrow rear end, or maybe wide rear pockets, but I don't actually ever sit on the knife unless I'm doing something like... Laying down on the carpet watching a movie or something. The knife is way off to the side. Also I've not yet come close to losing a knife, or damaging anything yet, but I could see how that could be an issue with some bulkier clips.
Also, I can kind of get peoples comments about pickpockets in big cities/crowds. I guess I'm just not the type of person that enjoys either of those things, so I don't think about preparing for something like that (I try to avoid big crowds and therefore cities as much as I can, I just prefer more space I guess).
Anyway, lots of interesting replies, its been enlightening so far.