I used to have a ruger 10/22 with those 10 round factory rotary mags.. I left them loaded for a few years, and they all had feed problems, and one must've broke or something, the ammo just fell out of the mag.
I've left my ak mags loaded (about 20 or so) for years, and never had a problem with one of them. I've left my hungarian .380 loaded for years, and no problems. I've left my hi point .45 loaded for as long as I've had it (maybe a year, not much, but hey) and have had no problem.
I think its just very poorly designed/faulty/cheap mags that have problems. I keep all my guns loaded to the very last bullet and have never had a problem other than that .22 I used to have.
And before that I had an oooold .22 that loaded just fine with the magazine it came with, even after I kept it loaded for a few years strait.