A common practice based on fiction. It's amazing how some of these stories get started then spread like wild fire. If a grunt hears it from some authority, he has no choice than to believe it, even if it's totally false, which this story is.
Any quality magazine, including the old GI 1911 mags have good springs that won't weaken, even if the fully loaded magazine is stashed for many years. I've never used the cheapies that come from China or Philippines so I can't say they are equally reliable.
If a gun won't cycle properly with a full magazine, there's something wrong with the gun. I carried a Colt Commander as a duty gun for many years. The duty ammo stayed in the magazines (two mags in a pouch and one in the gun) until I had to qualify. I would then use my duty ammo to qualify and load up with new ammo for duty. So twice a year the magazines got emptied and refilled a few times. Never once did I (or any of my fellow Agents) have a gun that failed from keeping the magazines fully loaded. The issue magazines were one Colt and two GI 1911s, all held 7 rounds.
What will damage a magazine is dropping it when it is fully loaded. The lips will bend and sometimes the back wall / side wall would split at the top. If you try to use a damaged magazine, it is sure to cause feeding problems. All you have to do is keep them clean (no oil) and inspect them carefully if they get dropped or abused.
Now that I'm retired, I use a Stainless Series 80 Commander for a carry gun. I use Wilson 8 round magazines and keep them fully loaded all the time. Never had a problem of any kind with them.
