I think if that's the way they want to use it, then it's fine. It's their choice. I do a bit of that myself, but not all day long. It's kinda like therapy, and it also lets people know you're a real person, so they can connect with you better. The key here is moderation.
twitter is a SOCIAL networking site, therefore, anyone has the liberty to post anything they like on the site any time and anywhere. It is another way for people to channel their thoughts and emotions to the world and I think we can all agree that somehow, sharing your ideas and feelings is a fulfilling activity.
I don't know how people find time to do all these work like twitter and face book updation.Might be they are free or they love to do it.