I do have two personal examples of springs weakening with use (from cycling, not from setting):
1. In 2006 I built up a .22-converted Walther P.38 with an unused (new) conversion set I bought off ePay. The mag spring was so stiff that the gun would not function with more than 5 rounds in it, and then only with super-peppy Remington Golden Bullets. After several hundred rounds it would function with six rounds in the mag, and then with seven, and then finally with all eight. After about 1000 rounds, it would run fine with eight rounds of Federal Bulk, which is less powerful. I've got 5000 rounds through it since then, and it's still working fine.
2. In 2007 I built up a 1911 with a Ciener conversion, and I used a brand-new USGI spec full-strength mainspring in it. This gun, too, would only function with Rem Goldens. Now, with about 14,000 rounds through it, it will function nicely with Fed Bulks. Full-powered ones, anyway. The problem is that most recent lots of Fed Bulk have a number of underpowered rounds in them, which of course will NOT run the thing, so I still have to use the (more expensive) Rem GBs in it. If the Fed Bulk quality ever comes back up, I'll be able to use them in it regularly.