Why is it alright to leave rifle mags loaded forever, but it is suggested to rotate Handgun mags every few months?

Just curious if it is myth or if there is factual basis for it

Some advocate loading your AR mags 2 rounds short. This serves two purposes, it saves the springs from developing a "set" and it allows you to seat a mag easier against a closed bolt. Ideally you should rotate AR mags just like your pistol mags, all springs can develop a set.

All of the Dozens of Rotate/Don't Rotate threads I have read contain this information sooner or later.

Quality springs do not develop a "set". Cheap springs can. Buy quality mags and don't worry about it. A spring made with the proper metal will not wear by being held in a particular position. It is the act of being compressed and decompressed that causes wear. Rotating mags causes more wear than leaving them loaded.

The issue with pistol magazines could be that there are more after-market junk mags to choose from, and some handgun manufacturers sell you a crappy mag right from the get go. Premium quality mags should contain springs that will not take a set.  

My on duty magazines are emptied once a year by fireing them then reloading with new duty ammo. They are almost 13 years old. They always function fine. I have other mags for training/practice. They also work fine. All of my mags are factory S&W mags for the M4516.
Now that I think about it, even though they work fine they are 13 years old. I should get some new mags.