Back pocket is wallet shaped and usually larger than front pockets, but yes it might not be comfortable to sit on. However, putting my wallet in my front, smaller, tighter pocket is no more comfortable sitting down...except now its not even comfortable standing up, something the back pocket never has any trouble with.
"It's easier to steal" has always come across as something of a strange argument, you would have to be a master to take something out of my back pocket without me noticing the difference and your average chav is no slight of hand expert. secondly, Is it REALLY that common that people steal out of your back pocket? Is it? It's never happened to me nor anyone i've ever known, to be honest it sounds like nothing more than a mothers tale - An over precautionary bullshit exaggerated story meant to make you overly paranoid.
Thirdly, I dont make a habit of carrying around huge wads of cash in my wallet, most of my money is on cards. Anyone who takes my wallet will find that all they've taken is a broken, shit faux leather wallet with £5 worth of coins, a bunch of useless cards and 2 very easily canceled debit cards.
Most people's phones are worth more than the immediate contents of their wallet, and most muggers know this. Guess where most people keep their phone? The front pocket. Guess when it doesn't matter what pocket you've put what in? During your common, garden variety mugging.