I've always heard that when repitching yeast that you should limit the number of times you repitch the yeast. Basically, that after x many (Between five and seven batches are the numbers that are coming to mind) batches, you're better off tossing the old yeast and starting fresh.
What yeast mechanism/health factor/issue drives this? Is it a mutation thing, where the yeast get further away from the starting point? Or do the old, unviable cells begin to take up more and more of the mass of the pitch? Potential for other critters to start to take over?
How could you "resurrect" an old yeast that has seen that many fermentations? Plate the yeast for single colonies and start over?