One thing I've learned from reading these threads is that a lot depends upon whatever magazine you are using. Different magazines behave in different ways. A 1911 pattern magazine that was manufactured before, say, 1950 has a relatively short heavy spring. A Glock Model 21 magazine manufactured after 1990 has a longer and comparatively lighter spring. I know from frequent use and lots of personal experience that these two different magazine springs are N0T going to function in the same way. I've had spring problems with Glock 45 ACP magazines that I never had with 1911 magazines.
I've, also, had problems with 30 round AR and AK magazine springs that I've never had with, let's say, Sako magazines or even SKS magazines. As far as all magazine springs being vulnerable to taking, 'sets' read the instructions that come with some of these high priced spring-powered air rifles. These manufacturers, all, indicate that their springs WILL hold a certain amount of compression (Think of it as, 'fatigue'.) after they are released.
Some of the best information on spring fatigue and the effects of prolonged compression comes from, 'JohnKSa'. (Who is, in my considered opinion, one of these Internet gun forums' most experienced and best minds.) You can google any number of John's comments, if you like. There are a number of articles; but, here is the one I like best:
The idea that springs don't weaken from being left compressed is only true in the ideal case. A well designed spring, properly manufactured from good materials that is not over-compressed will not weaken from being left compressed. When you change any of those factors you end up with a spring that can weaken from being left compressed.
This has been common knowledge in the air gun world for many decades. Spring piston air gun springs are typically over-compressed when the gun is cocked because that offers the best combination of ease of cocking, performance, size and weight. They could make the springs so that they weren't over-compressed when cocked but that would either result in a much larger gun, a much heavier gun, one that's much harder to cock or one that has poor power for its size.
To optimize all those parameters it's usually considered acceptable to sacrifice spring life. Particularly since springs aren't that difficult to replace nor are they terribly expensive. At any rate most any spring piston air gun comes with a caution that the gun should be not be left cocked (spring compressed) for long periods of time because it will weaken the spring. For doubters, tests have been done showing that the longer the gun is left cocked, the weaker the spring becomes, I can dig up a link if anyone cares that much.
The same applies to magazines. Good quality typical single stack magazines don't seem to have this issue, but sometimes when a designer tries for maximum capacity, minimum size without pushing loading effort off the chart, the spring is what must be compromised. I'm sure the designer was telling himself that it was no big deal because mag springs are very inexpensive and simple to replace.
(09/26/08: JohnKSa/TheFiringLine Forums > Hogan's Alley > Handguns: The Semiauto Forum/Does Storing A Magazine Loaded Degrade The Quality?)