My G21 Duty mags stay loaded 24/7
I used to use them for competition too, so they would get unloaded and reloaded with ball ammo for the match, then reloaded with Hydrashock at the end.
Now I have a extra set of mags, so the only time the duty mags get unloaded is during qualification (use the old duty ammo to qual with).
I have never had a problem with them and they get the crap beaten out of them on my belt.
My AR15 mags stay loaded constantly as well, but department policy is to download by two. So I keep 28 in my 30 rounders 24/7.