I have checked on this from time to time to see if anything really sticks out on this.
Google Base seems to be a way to put content on the web if a) you don't have a website and/or b) you want to wrap some meta-data around it.
Option a seems silly to me. You can get a blog site for free which requires NO knowledge to setup and run. Hey, Google has such a site itself. Anyone tech-savvy enough to surf the net can use Blogger.
Option b also seems a little silly. Microformats provide a way to add some meta data. For some of the things, like recipes or want ads, why not create a microformat to facilitate searching?
Looking through some of there posted items you see things like Events & Activities (similar to Upcoming and classifieds like Craigslist.
Now the classified idea is not bad since Craigslist does not cover all cities, but this feels like a "me too" type of service. Google, much like Microsoft or Yahoo, wants to provide ALL your data services. Many of the guys at Off The Hook agree that having that much data concentrated at one spot is not a good idea. I have to agree. Google maybe good about security, but as we have seen recently in the news even some of the best can be compromised. Think about what would happen if someone took your computer. Think about what would happen if someone broke into Google (or any of the others) and got all your personal data.