I don't think it has much use except as a browsing tool.

On the other hand, I think there's a big difference between browsing other people's aggregates (delicious & flickr) and browsing your own stuff.

For example, if I'm looking through my 'documents' folder for a file I know I worked on, it would be awfully useful to be able to, with a few swift clicks, whittle down the selection by what I may have tagged it with.

I wrote a bit about this and then realised that it sounded awfully similar to what MS seems to be doing with WinFS. So who knows...