Blogging is fun.
That is not the reason you should be blogging.
You should blog because:
1. The blog format lends itself to constantly updated, keyword rich content, which search engines love
2. blogging can help connect you with your clients
3. blogging helps you become an authority in the community, in relation to clients and other businesspeople.
I sometimes lose track of SEO being the #1 reason to blog, because the community aspects of it are so exciting. But we need to keep that in mind: most of the clicks go to the top 3 spots in Google. Getting in those top rankings is more important, for most clients, than building a strong community.
Community building is what you do once you already have the top spots in google and you want to grow even more.
(cribbed in part from conversations with Ian, not that he would say it this way)