Back in the day I used to leave my 92fs mags loaded all the time without worry, so I'm guessing Uncle Sam must have found a really good deal on spring steel that was not heat treated properly. This is another reason why I won't work for the Gov. The only companies that recommend rotating the mags often that I'm aware of is Walther and HK.That recommendation on monthly magazine rotation may have something to do with the idea that leaving them compressed will weaken the springs. Springs actually weaken from compression and decompression--not just from compression. Of course, that is contingent upon the springs being comprised of quality material that is designed for the amount of compression to which they'll be subjected.
I remember my first trip downrange in '03 and having rounds fall out of my M-9 magazines because the springs had weakened in only a few weeks. That became a common complaint amongst people assigned M-9s, and the DoD switched to a different manufacturer. I didn't have the same problems with the magazines during either of my later deployments. The bottom line is that the steel used for those springs was not designed to maintain its structural integrity when compressed to the point that the springs had to be compressed for a fully loaded magazine.
Walther makes higher quality magazines than that, so you won't have springs weakening on you. With that being said, rotating the magazines is a good way to ensure that you don't get gunk built up in them. It is not uncommon to have a lot of dust an other stuff accumulate in your carry magazines, and when you do the rotation process, you forestall that accumulation.