I've been suspecting this for some time, and even pondered posting about it a few months ago. The fact that I never got around to it only supports my opinion:
Blogs, especially writer's blogs, are a waste of time.
The point of writing is to write. If you get too caught up in the 'on-line community' you end up spending far too much of your limited time and resources writing your blog, reading other people's blogs and posting to various forums.
Some time ago I proposed to develop a circuit--a group of blogs and forums I could surf around, reading and posting to--as a means of promoting my own blog and web site. And do you know what? I stopped after a few weeks because there was just too little time for it.
I used to faithfully start posts for this blog, only to have them turn into articles; articles I could potentially sell (and I have actually sold a few). Why should I give it all away for free?
I have just had a look at some of the blogs I used to check--many run by professional writers--and some of them have signed off, having realized what I did some time ago: there just aren't enough hours in a day to produce good, saleable work and still connect with all the people out there worth connecting with. The fact that they are worth it is irrelevant; there are simply too many of them.
I'm not signing off. I don't need to; I hardly ever post anyway. I still expect to post my milestones (have I mentioned I sold a piece to Writer's Digest?) but in the mean time I think I'll take a break from the Internet and write an article about how blogging interferes with writing.