I've posted this before, but what the heck:
Metal springs weaken over time due to metal fatigue. Metal fatigue is the term we use to explain the microscopic cracks that develop in the crystalline structure of the metal. Those cracks are created by repetitive deformation of the metal. As the metal is deformed the cracks become worse. The more repeated the metal is deformed, the worse the cracks will get. Eventually the metal's ability to return to shape weakens. Unfortunately there is a lot of mythology in the firearms community about how metal fatigue works. Most of the mythology points to leaving metal springs compressed over long periods of time. Except that the function of metal fatigue requires repeated deformation. Once a metal spring is compressed it does not continue to form the fractures. The methodology of the fracturing requires compression followed by extension followed by compression etc, etc.
In other words, compressing a spring for long periods of time will have negligible affect on the structural integrity of the spring unless you hyper extend the spring past it's rebound point, which is called spring creep. What actually weakens springs is the constant action of compressing and then releasing the spring, compressing and releasing, etc, etc. This is called cyclic loading. When you load and unload a magazine you are introducing and exacerbating microscopic fractures. That's the only time when those fractures occur. Leaving a magazine loaded will have far less impact on metal fatigue than cycling the spring frequently. Fatigue is irreversible and cumulative. Once it's started, it will continue and accumulate over time as you compress and relax the spring.
This might help people interested in understand the mechanism of metal fatigue:
http://www.epi-eng.com/mechanical_engineering_basics/fatigue_in_metals.htm
http://larrylawson.net/fatigue.htm
If you're really a glutton for punishment you could read:
ASM Handbook Volume 19, Fatigue And Fracture
But generally only structural engineers read that kind of stuff.