Twitter may not have direct seo benefits, but it still has benefits in other ways. It helps you overall online presence and it allows you to connect with others. I was quite dismissive of it thinking that it won't apply to my main business (which is a localised service business), but then I connected with a few people locally through Twitter, which led to face to face meetings. So, now I see it as a tool amongst many. Just another way to meet potential new clients.
However, the only negative, I would add is that, social networking, or marketing, requires a lot of time investment - most of forget how much. If you put a value on your time then it can be very expensive.