It's not appropriate to try to stream this incremental info with keyword searches. It just doesn't work. Say you want a feed of interesting news about Google. A while back I posted something on this blog about Google which you'd probably want to see in such a feed. But the rest of the articles here are not about Google. So you don't want to subscribe to blog.topix if you just want news about Google. But a keyword search for "google" isn't going to deliver a useful experience either -- there are far too many stray mentions of "google" on the web every day. To get a relevant news feed about Google, you either have to have people read everything for you and edit away all the junk, or find an algorithmic technique to do the same.
I agree that simple keyword searches may not be the best, but keywords and tags can be one of several methods to define the subject of interest or the topic to subscribe to - it's the subject or topic that's the slippery beast.