I’d say that hashtags can provide extremely valuable meta-information, because they signal both intent, as well as a certain amount of sophistication/insider information (compared to “incidental”/unconscious use of the same keywords).

E.g. during SXSW Interactive, Twitter’s time-line on Search.twitter.com was overflowing with SXSW references, and cutting that down by searching for the #sxsw hashtag only was a first step toward cutting through the noise.

And the great thing is that they can be added to the text flow without needing to repeat the term twice. E.g. you can just write: “This post deals with the way #wordpress handles…”. What’s not to like? Only 1 character need be added…

Agreed that people need to avoid abusing them by making anything a hashtag. Basic rules for viral “spreadability” still apply: Keep it short, logical/meaningful, easy/enjoyable to say (“#followfriday”) , etc.

But if used properly, each tweet using them adds an entire additional dimension of communication opportunities, because a vast number of people monitoring Twitter, who are not yet your followers, can find you via hashtags (that number will only grow with increasing Twitter Search literacy coming to the mainstream).

You can join a much larger conversation. Also see the success of #journchat, #brandchat, etc. as virtual, timebound meetups on Twitter.