This is exactly why I take in old Macs I come across. Yes, I have several unused Classics and SEs sitting around but they could prove to be valuable if a) one of my in-use ones dies or B) someone else needs a part that only I have.

If I get a marginally useful Mac (like a Classic I got from a friend last year that had a flaky analog board) I will usually let it sit until I need a particular part. For example, I may pull the floppy from that one or strip it of its RAM if I need either of them.

Don't forget that hard drives fail too. Especially old ones. When you get a new Mac in, make sure you use a good drive wiping program on the drive to sort out the bad blocks--the "Erase Disk" command won't cut the mustard.