I happen to think it's only unfortunate when the caching proxy is set up poorly, but I'm guessing that's more often than I'd care to know. If AOL didn't cache, I think many of us would be using a lot more bandwidth.
Course, some things you absolutely don't want to be cached, but a LARGE portion of web content that SHOULD be highly cachable, isn't, either because folks haven't gone to the trouble of making their dynamic pages cachable (I've been guilty of this) or because the server settings are just poor. (One host I was on didn't have LastModified dates sent along for static pages and graphics. Why on earth I don't know, but it meant a LOT of useless traffic, particularly of the gifs and jpgs for the site)
Let's not even go into Expires headers or dynamic content returning 304s where appropriate.