There are many benefits to use wordpress MU version. Benefits are from the host side as well as from the user side. These are some of them:
HOST SIDE BENEFITS:
* Ability to create a hosted “community” of users.
* Ability to host thousands of users on one site.
* Opportunity to provide free hosting as an alternative to paid hosting services.
* Ability to limit sign-up to specific email domains if required.
* Various options for adding plugins and administration features.
* Customization of the administration panels.
* Ability to add WordPress Themes, increasing user choices.
* Ability to control WordPress Themes, limiting user choices per site needs.
* Security precautions and protections in place.
* Easy addition of comment spam protection through plugins.
USER SIDE BENEFITS:
While WordPress MU users are blogging with a limited version of WordPress and currently are able to choose from a list of included WordPress Themes and have little customization there are still a lot of benefits.
* No installation or setup. Enter user name and password and blog.
* A chance to test drive WordPress at little or no risk.
* Choose WordPress Themes that have been tested and are ready for use.
* Part of a community with potential for community relationships through competition and awareness.
* Comment spam protection built-in.