Tags are good! Use them! :) Here's a nice overview of tags: http://telligent.com/support/telligent_evolution_platform/community/w/community6/19847.about-tags.aspx
Post title, post body, post attachments and tags all factor into search. If a term you entered matches a tag, content tagged with that term is part of the relevancy scoring.
Tags are going to help cross-relate content as well. So, if you click on a tag in the tag cloud, your community members will find content with the same tag.
We also have a really nice tagged content list widget. With this, you can list blog posts, forum threads, etc matching the tag you specify in the widget configuration.
BTW, in 7.0, we added hashtag support. This is a nice quick way for community members to tag status messages, posts and comments.
So, there are lots of things you can do with tags.