There is a plugin on the ProBoards store that integrates with the search engine, causing #Hashtags to be searchable in posts, just like the ones used on Twitter and Facebook; if this was available on your forum as a plugin, or rather a core feature, would you use it?
What about without the plugin, do you use hashtags in posts, or rather see members using hashtags in posts.
I'd definitely use it. Quite a few members would definitely use it, and I don't see a reason why it shouldn't be added if available, unless it would somehow annoy your members, or you already have a crap ton of plugins installed already. 😛
Even for small communities you can attract post idea's and what not with hashtags, I honestly think they would make a smaller community more active considering your members are used to the Twitter theme that is hashtags.