Two points about rotating carry ammo. My father has a friend, a retired small town police chief, who carried a Colt Python (beautiful) as his duty gun. He swore that, based on personal experience, bullet lube will melt from heat, especially on a hot summer sunny day, and can contaminate the powder resulting in a squib shot or no shot at all. He made a habit out of changing ammo at least weekely in the summer. Of course, he only needed to swap out 18 rounds, so it wasn't as big a deal as it is now with the high-cap autos.
Second, my concern about the chambered round is less about the possibility of round set-back, and more about the rim getting chewed up too much. I recall reading about this back in my younger shooting days pre-internet when I read the gun magazines. And, I've noticed a roughness to the rim on some rounds I've chambered/ejected many times. It may not be a big concern, but I am worried if a burr develops that keeps the round from properly engaging the extractor, or getting humg up because of the burr.
Personally, I try to swap out the chambered round about once a month, but leave the magazines full until shooting time. Even then, I've often loaded and unloaded carry ammo a few times over a year or two. With the price of carry ammo, and my shooting budget getting hit by gas prices and other expenses, it may end up the same or even longer...
