It's somewhat depressing, but probably not surprising to anyone working with educational technology. Students also have to be prodded to write discussion board posts, turn in homework, take quizzes and read the material (via check-up quizzes).
The temptation is to blame the modern educational environment, but I really think it all comes down to another important educational factor ... motivation. As many of us know, motivation is divided into "extrinsic" (motivated by certain social goals, like a good GPA) vs. "intrinsic" (true interest or "The Love").
Blogging is fun if you are interested in the topic. If it's another "essay" though, it's not as much fun...it's just work. When a course is taken to fulfill a requirement or fill out a schedule, the motivation to do more isn't as strong.
It's only you are in a course which you take for the heck of it, you may actually be willing to read the extra readings, do the extra exercises or even read items completely unassociated with your class. I actually did do the extra exercises in intro linguistics (and didn't even turn them in). Did I do the extra exercises for statistics? Not really.
I wish all courses could be equally interesting, but I don't think that's a realistic possibility. Why should one instructor be interested in the bombadeer beetle and another in Ancient Athenian politics and drama? The beauty of the academic experience is that there are so many different ideas to obsess over - no one could ever possibly obsess equally.
So is blogging in the classroom a waste of time? I don't think it is. If nothing else, a well-formed blog assignment may force the student to think about an issue in a new way. Sometimes, an assignment is about giving the student the "opportunity" to practice a new skill...whether they want to or not. And you never know...intrinsic interest may come later in life.
You would not know it from the amount I blog now, but there was a time when I found most writing courses very tedious. In retrospect, I feel sympathy for my poor instructors who were probably frustrated that I couldn't find literature as interesting as I found Indo-European. Oh well.
One assignment I was particularly annoyed at was one based on the Dante's Inferno where I had to define my "personal circle of hell." It should have been fun, but I just wasn't up to the introspective challenge, and my classmates did a much better job than I did. Years later though, I finally was in a situation where I was able to define it...and then it was so funny I had to laugh at myself. If I hadn't had this assignment though, the "answer" would never have come to me.
P.S. My "personal circle of hell" is an eternity where I move my pile of books to another room, then I move them back to the original location in an endless cycle.