Well, I do, but you're not the primary reason why I do it. Though readers are nice to have, and I appreciate that 20 or so that struggle their way through my posts, what I've found most rewardingis that I've learned so much while blogging. What may have taken me three or four YEARS to take the time to learn before, I've learned in the last two months as I've tried to keep up with my blog.
If you've ever thought of doing it before, and you've been discouraged by the fact that you don't have anything unique to offer the blogosphere, do it anyway. Who cares if someone has posted on the subject already. Who cares if the niche that interests you is overflowing with bloggers. I'll guarantee that you'll learn a ton about your subject if you blog about it anyway.
In addition, I'll guarantee that you'll get a small number of readers. Whether they are colleagues, friends, family, or pets, they'll put just enough pressure on you to try and keep up with it. And if you keep up with it, you will learn.
I suggest picking a subject that you're interested. Pick a topic that you are not an expert in, and write a tutorial. You'll learn a ton. There's two sure fire ways to become a better programmer, read lots about programming, and write lots about programming.
If you decide to do it, let me know. I'd love to read it.