First off: I'm a total n00b. I was cleaning a bike that I got from my mommy, and noticed a cracked bit on my rear derailleur. It seems to be shifting fine, but I'm (perhaps unjustifiably?) worried that this will cause all sorts of problems. The Altus LT doesn't match the rest of my groupset anyways (Shimano 600) -- is this a good excuse to get new derailleurs? Or can I hold off for a while?