A lot of wordpress themes are butt ugly. Set up by seo guys to get backlinks. So I think its better to have a much smaller amount of themes - only the ones that look good. Then overall your Mu install will look good.
So what are your favourite themes?
Also, how much overhead does having all these different themes put on your MU install?
It would seem most themes could be coded using sandbox and css. Then you could just have a sub menu in the 'Design' menu which allowed you to select a skin for sandbox.
That way whenever you had some major update like avatars to wordpress you wouldn't need to go through and change all the themes just change a few different sandboxes.