Should google be indexing individual posts in a forum?

[Mr Lucky]

Obviously google is indexing individual posts, because I can see it is. But is this good or bad for SEO?

This post shows up quite near the top of results, but I would have thought it would be more useful for the thread to be indexed not post #7, which actually doesn't contain any of the search terms . In fact the text snippets quotes post #2



I thought this was the point of robots.txt

Disallow: /posts/

Although I'm aware that doesn't stop Google doing anything. In spite of this:

The robots.txt file, for example, prevents posts from being indexed, unlike threads which are. This avoids diluting the good information of your forum in thousands, tens of thousands, millions of posts.

Any thoughts ?