How big of a deal is this broken bit on my rear derailleur?
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?
That looks to me to be a decorative cover over the cage spring. Is it plastic? If so, it probably should be fine unless it got broken due to a more serious underlying issue. If the cage rotates and springs back properly and doesn't have excessive play in and out, it's OK.
the plastic piece is not structural; it just keeps dirt out of the spring. your derailer is very old (early 1980's I think) but if it works fine keep using it. if not, it's very easy to find a cheap replacement in the leftovers bin at a bike shop or bike co-op, since your system is not indexed.
I had an old Shimano Z-series derailleur with a similar part that cracked. One day in the driveway I heard a "PING" as it shattered and the chain went slack. It seemed like that part had a function related to keeping the spring wound up. I never found any other part so I think that cover is actually important. Based on that experience I think you are on borrowed time and should probably get a new derailleur.