WordPress MU

As the question about integrating WP an WPmu (http://mu.wordpress.org/forums/topic.php?id=8040) turned into a deadend, let me rephrase the question:
- I currently have a WP blog, with several authors and contributors
- the existing blog will continue to be the master/main blog
- I would like to offer the users, to make their own blog and also offer team/group blogs as well
- WPmu seems to be a great solution for this ... that's why I'm here anyway :-D

So the question is:
- what are the benefits of importing the existing blog, to become the mainsite for the WPmu installation?
- versus importing the existing blog to be the subdomain "www"?
- the main-blog needs to have a special theme and special plugins, not available to the user blogs. Is this doable, both the mainsite way and the www-subdomain way?

Plugins - I keep the plugin directory hidden for all except the Site Admin (me) so the users can't see or play with the plugins anyway - so in my case yes you can have some plugins that only the main site will have.

Themes - Yes - you can enable themes to be available across all blogs, and also enable themes on a blog by blog basis - so you can keep your main sites theme private.

If you are going to the trouble of installing and maintaining MU, then you may as well import and run the existing blog using it as well. It will at least save you a little bit of time than with a separate WordPress install. Other than that there isn't really any benefit that I can see.

I run all of my blogs on MU now, as it is so much easier and quicker to create a new blog rather than messing with a new install, wp-config, database, etc...