With WordPress we took blogging to the next level by creating powerful self-hosted software that anyone could install and have a blog within minutes.
Now with MU we’re making it as easy to create a thousand blogs as it was to create a single blog with WordPress 3 years ago. By building on the base of WordPress and syncing development, we get the benefit of the huge ecosystem in plugins and themes that has developed around the project.
Development on MU is spearheaded by Automattic, since they use MU to power WordPress.com a great deal of the companies resources are poured into improving the code, and it all goes back into the open source world via MU.
If you’d like some background on the WordPress project, check out its about page. Since WordPress and WordPress MU share 99% of the same codebase, the features and development on one also benefit another. As WordPress is used by everyone from Yahoo to the New York Times, it isn’t going anywhere.