I keep stuff in my pockets all the time... my ePhone, wallet, a snack, tools, inner tube, etc, especially if I am riding a bike with limited luggage carrying capability. I have never had a problem with this practice. However, last week I went for a rip with my friend's bro and he was carrying a multitool, a tube, and a banana in his jersey pockets, and all were ejected over a 20 km ride on rough roads. His pockets were a little shallower than normal, I think.