I think hashtags are silly, mostly because they make tweets harder to read. Especially hashtags that are the punchlines to jokes, since they're usually a phrase that I have parse out. (Don't get me started on tweets with multiple links, @-replies and hashtags.) I can get behind some hashtags though:
hashtags for that conference you're currently attending, so I can filter out your tweets about it
memes like #BlueJaysMovies or #urbanskillscompetition where we can make punny jokes or clever ideas that'll never fly. Other memes like #firstflickr warm my heart.
I only ever use hashtags to tag tweets, not as words in a sentence. So you can filter them out! And so you can read them!