Since a lot of what I do is teaching writing, I subscribed to a search on the topic. I have been interested to see what I discover this way.
Probably the most interesting so far has been a reference to this article by Philip Pullman on the "basics" of teaching writing. He says, among other things, that we should should allow children to play with language rather than beating them up with the rules. Now, he isn't talking about ESL or EFL students, but I think it would hold true for them as well. Pullman also states that teachers must be "confident about writing - about play, about delight. Too many are not, because they haven't had to be..." If you are interested in teaching writing, I highly recommend the article.
I think that Pullman's thoughts lend credence to the idea of blogging. If students are captivated by technology or if they have trouble putting pen to paper for some reason, blogging could be considered "play". It is less threatening, I think than traditional writing activities. Also, the study he talks about found that "giving them formal instruction in grammar turns out not to be of any use; getting them actually writing seems to help a great deal more." So maybe assigning a certain number of blog posts a week runs contrary to the freedom of the Internet or something, but it would not seem to run contrary to good pedagogic practice.
Read the article. See what you think!