One of the things is that it reduces the "branding" of your community. When people look at the community the first thing they notice is the visual aspect and appeal of the site. Only then do they observe content.
I've seen dozens of communities that use multiple visual themes selectable from the browser. This is good for a forum styles development community but not for a regular, discussion community. People need some visual indication as to your community and having multiple selectable themes will reduce the "recall" factor of a website.
I personally think that any community website should have something unique about it and try and cash in on that rather than trying to satisfy everybody's idea of what looks good. Capturing a niche audience means that you cannot cater to everybody's tastes.
So what are your thoughts on this? Do you think you improve a community by having multiple styles?