I agree with your sentiment, Jay, but I have to say I cut writers on Twitter a healthy amount of slack. They are by far more interesting and better spoken than celebrities and sports stars who take to Twitter.
Writing is a solitary task, and to me, writers' tweets are like characters' internal monologue, distinct from dialogue. They are putting it out there, but it will probably only be read by people who do value their thought processes. It's strangely encouraging to know that my favorite writers have the same random thoughts and songs playing in their heads that I do.
I agree that one should always keep in mind what to put in and what to leave out, be it formal or casual conversation, but it's difficult for me to get overloaded on Twitter when there are only 140 characters per tweet and I can't possibly read every tweet from everyone I follow on any given day. Tweets are, as Hemingway might say, glances at the tips of the icebergs that are people's lives.
Three years ago, I was reluctant to join Twitter myself. I'd had this blog for four years at the time and didn't see the need to join a micro-blogging service. I didn't use a cell phone day-to-day (still don't), so I couldn't report from intriguing locations.
My brother, who quickly gave up traditional blogging, claimed Twitter's brevity was perfect for him. I wasn't convinced until I learned I could link my blog's RSS feed to Twitter and reach a whole new audience.
Both of us are still on Twitter today. It's not for everyone, of course. The trick is knowing whether it's for you.