I'm not sure what advantage this has. It could be used to make a self-updating Greasemonkey script- by simply changing the files on the server, every user that uses the script would now benefit from it. The obvious downside is that this newly loaded script could be changed on the fly- without the user's consent. It still runs in a sandbox, but I can easily use DOM objects like the IFrame to break out of it and do... well, nasty nasty things.
I think though, a sever-side GM script could be really useful for multi-user applications. Perhaps I'll write my client-side loader to do some basic validation and force the user to see the script, or verify a digital signature or something. I'll have to poke around on the GM and Firefox APIs to see if that's feasible.
One use that I'll have even before adding the security is for pushing scripts between home and work- only one script needs to be kept up to date, as opposed to a dozen.