I'm inclined to believe that leaving a magazine loaded for a long period of time won't harm the magazine, but it may harm your ammo. Brass is a soft metal and over time the constant pressure of the follower pushing it into the feed lips of the magazine could deform the brass and possibly even create a weak spot. I believe I read about this theory on the CCW forum over at GlockTalk. The concern was with U.S. citizens that leave a loaded pistol in their bedside drawer month after month, year after year and never shoot the thing - probably not a big concern for most of us Canucks.