Twitter is a waste of time…. unless you use it in a way that isn’t.
I would argue that there’s never been a type of communication invented that can’t be used in a way that is a waste of time.
Twitter can be beneficial to bloggers in many ways (I’ve written 9 ways that Twitter benefits me here) but the key is to work out how you want it to benefit you and to go about using the tool (and it’s nothing more than a tool) in a way that takes you a step closer to it paying off in those ways.