How worn can my equipment be before it really needs replacement?

Knox and I had a little discussion offline which covered jockey wheels. To illustrate how badly worn my jockey wheels were (and, I suppose, how bad I am at maintenance - Roger at Idealbikes calls my style "ride it into the ground"), here is what my new jockey wheel looks like:



And, for comparison, my rather spiky worn out jockey wheels:



Surprisingly enough, these wheels were still mostly working when they came off the bike.