You can knock it, but back at one point the only way to get "free" website hosting was to be on Geocities. My brother and I started up our first websites on Geocities (actually, we shared one) and eventually it grew so popular that we had to pay Geocities to get rid of the annoying ads, we had our content stolen and mirrored and eventually had to move onto our own hosting (which gets more bandwidth taken up each year). That site now stands proudly at www.scoutingresources.org.uk and stresses the (very kind) host that it's on at regular intervals!
Geocities was a seed. Yeah, there was a lot of crap, but that statement can be applied to the Internet as a whole. What it did, though, was seed the ideas of HTML and putting content online to a whole host of people who couldn't afford hosting, or didn't know if hosting was for them. It greatly simplified things like forums, etc. and I'm not saying they were the first, but they had a good community and if you were just starting out, it allowed you to do *just* HTML without learning FTP, Unix permissions and a ton of other stuff. It was a fantastic intro and testbed.
I don't even know if our original website is still up, (it was mirrored and archive.org'd, though, that's for sure) but I would like to think that some of the stuff in my old bookmarks that happened to be on Geocities would stand a chance of surviving. It's a bit of Internet history, even if only for lessons learned, and one of the first real social networking sites (think about it - webrings, forums, "street names", it had everything needed to be a social networking site, not just a HTML host).
Even today, I'm porting code from a GBA port of a Spectrum game (http://www.geocities.com/quirky_2k1/) - all of which is hosted on Geocities and would be a great loss if it were to disappear.
Not everything useful has to come in a leather bound binder, with top quality paper. Similarly, not every website has to be on www.website-topic.com to be relevant or useful. Things like circuit diagrams, pinouts, obscure information on machine internals - I often run onto Geocities while hunting down this type of vital information and to lose it just because it was Geocities would be stupid.