I believe Manco and Drail are correct.
here is my experience;
I had an xd40 with 12+1 capacity I carried as a security guard. when I bought it with three mags, the springs were all VERY stiff. Difficult to load. I left two of them full to the brim for 3 months with out touching them before I heard this so called "Myth". Now, I have been called a liar, dead wrong, and an idiot, but I am telling you the truth when I say, I noticed the springs on those two mags were lighter and easier to load at the end of those three months than they were when I first got them. At that point I decided to rotate them through on a weekly basis and only load 10 rounds instead of 12.(no one had told me rotating them wears them out also at that point) You can take that to mean what you will. Could their be other reasons for my findings? very possible, But the facts are I had mags with stiff springs, left them full for 3 months, with out being unloaded, then they had softer springs.
Now, after I quit that job I ended up shooting that gun quite a bit, and I never had any jams, feeding issues, or malfunctions with any of the three mags, But I do believe in specific applications it will soften the springs. Would it ever be bad enough to cause a malfunction? I couldn't say. It does not seem to affect 1911's.
My understanding is with tube magazines in shotguns, its not the spring you have to worry about, but the shells. They get compressed length wise causing a "bulge" in their diameter and then won't chamber.