Not really, no.

My ex-wife always sat with her 3 in her back jeans pocket because fashion designers refuse to put sensible front pockets on women's jeans. The 3's case began to crack at the top around the power button after a while. And this is a girl sitting on her phone - not some 200lbs guy.

And I certainly wouldn't sit on my nexus 5 or the SGS4 that it replaced.

Eh. I (until recently) weighed around 220lbs. I would not have been surprised if any phone I sat on [0] broke or was otherwise damaged. Especially with the way that anything in a back pocket gets flexed. It's not just the weight placed on it, it's the tension from conforming to the shape of your body along with the pants. I guess if I had no ass, or weighed much less, or like some coworkers wear pants so low that the back pocket is actually behind the thigh it might not be a big deal.

[0] This also depends on the seat. My couch at home is very cushy, it'd be hard to break it there. But my kitchen table chair is little more than a slat of wood cut to a comfortable shape.

I'm 185 pounds, and I'm absolutely confident that if I ever placed any of my iPhones in my back jeans pocket and sat on it, it would be destroyed in under a month.

It has a glass screen, that alone should make it obvious that you shouldn't sit on it.