Yeah but all your cycled discs don't do you any good if you change sponsors. During competition you want the disc and muscle memory to do all the work because less variables is better. To create the muscle memory and keep your skills honed, you monkey with discs when you're not competing. It's about the difference between being able to hit your lines with that Teebird and being able to hit your lines with all your Teebirds/Teebird clones.
The pros can go to a factory and pick out a group of similar discs, cycle them, and pretty much have a group of near identical discs. A huge advantage for the touring pro. But sure.. I would guess in the off season they play some random rounds to fine tune their dynamic ability to adjust to various conditions, lines, etc. But once the season begins, it's back to the bread and butter discs.