Interesting, and not something I've thought about too much. It could be useful, particularly for conveniently identifying which Space you are currently in (if you don't use/like the icon in the menu bar), but I don't know how much I'd use it. Then again, I've thought that sort thing before!
In previous years, I remember Linux users telling me about virtual desktops, but I'd never been too interested. I tended to use the Hide command for any application that I was not currently working on, so it didn't seem too relevant to me.
When Leopard was announced and its features detailed, again, I wasn't too bothered about Spaces. "Nice to have, but I'll never use it" is what I thought.
When I installed Leopard, I did play with Spaces and felt it may be useful after all... Once I configured one of my mouse buttons to activate Spaces, made more Spaces available (I usually use nine), and configured the System Preferences to make certain, regularly-used apps (e.g. Mail, iTunes, Terminal, etc.) load into particular Spaces, then I was rather hooked! Now I wouldn't go back to working without it.
Ah, but I digress...