UPDATE:
okay, i'm developing multisites with 2 main thoughts:
1. a site admin would want this capability, to easily clone sites and reduce administration worries
2. each site wouldn't necessarily need unique data, so in my own use, i'm implementing small modifications to modules and their database tables, referencing a 'sid' value (site id), the site id is defined in each site's mainfile.
when i originally developed my multisite mod, i followed the same line of thinking that already existed, which meant that a ton of database tables were getting replicated, which in practice, seemed a bit useless to me. i also still had the same problems with administrating multiple sites, since in many ways, the sites were still separate.
well, now i have ALL tables uniformly shared (except for the sessions table. i need to make a slight mod to that, to use the 'sid' value, and i'll be good to go). i even have the config tables shared, and xoops_config has a new conf_sid value, so that certain entries can be assigned to certain sites.
the only implementation of conf_sid that i have in place at the moment, is a mod so that any value with conf_catid 1 or 3, pulls data specific to it's own site (so, i can have sites with different settings for site name, theme, etc, and also the site's meta settings)
this greatly eases module and site administration, as i'm now able to change something like user settings, and it take effect across all sites.
also, i can now install a module on one site, and it'll install across all sites (and eventually, i'll implement the sid mod further, so that i can install modules individually on certain sites)
another area this is useful in, i've modified wf-downloads and a few core XOOPS functions, to use sid when necessary. so, for wf-downloads frontend, i have downloads and/or categories only show to the users for it's associated site, yet from the backend, i can manage all downloads for any site.
very nice