I reserve my back pocket for "beaters," knives I could stand to damage or lose. They really do all the cutting, as the nicer folder in my front pocket is basically a toy that only comes out for drills and hopefully never for the other reason (which is why it must stay sharp and close-at-hand).
Back pocket access is a tad slower, and big knives might be uncomfortable to sit on, but I've never done one much damage, though the clip gets roughed up a lot. The one problem I have frequently seen is that particular couches and such will steal my knife by hooking the clip when I sit down - if I slide down into a lazy position, my knife stays with the furniture!
I have a customer who carries a titanium-scaled AFCK that I "made" in his back pocket everywhere he goes. I admit that I expected sitting on such a big folder all the time would be bad for both user and knife, but every time I see him he shows me the knife and it looks fine. I can only assume he's comfortable, too.
I wouldn't recommend the back pocket for fastest access or for storing a big or expensive knife, but I've yet to see a knife damaged by this carry.
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-Corduroy
(Why else would a bear want a pocket?)