I just read the strangest apologia for a new service: Uh, why’s the official Tumblr blog on WordPress? (broken link - http://blog.davidville.com/2007/02/23/why-wordpress/#comments)
Simply - all the CMSy stuff it comes with. Blogs are an awesome platform. WordPress lets our entire staff contribute to the same blog, maintain tags and slugs, save and give feedback on drafts, upload and store media, back and forward publish posts, group our archive by month, lets our audience comment, lists trackbacks, et cetera, et cetera. It’s awesome! Blogs rock! But we knew this. WordPress is the perfect way for a business like ours to communicate with our audience.
Sounds good to me. David Karp goes on to write about the advantages of Tumblr: "posting with zero obligations, little or no comment". Great for wisecracking, difficult for communicating.