From my point of view, the primary advantages of the server/client setup over the NAS/separate libraries model are:
1) Only having to configure or change views in one place; if you have a lot of custom views it can be nice to only have to change them once and have it propagate out to the rest of the house.
2) It's much easier to share library metadata that doesn't get written to files or sidecars. Certain library fields aren't written to files (or can't be), so only exist in the library itself, see the chart on the wiki
http://wiki.jriver.com/index.php/Tags. Probably the most significant one of those that I run up against is the "number of plays" tag. If you want to know how many times a given file has been played, or if you record a lot of TV (and want to know what's already been watched), sharing a single library is super helpful. Otherwise I tend to lose track of what's been watched (and can be deleted) and what hasn't been watched.
Some disadvantages are:
1) The server must be on (or at least sleeping) in order for clients to have access. If you have a PC that isn't super power hungry, or if you can get wake on LAN working correctly, this isn't much of an issue except after power outages. I've had some wake-on-LAN issues with my hardware, so I just leave my server on.
2) Certain types of library maintenance activities can only be done from the server. Changing library views or importing new files have to be done on the master library; Adding cover art or ripping CDs either have to be done on the master library or in special purpose libraries on clients, but the latter takes some fiddling. Since you currently have a NAS-based system, this may not be that different than what you're already doing for library maintenance, but it's a definite limitation of the current server/client model.
For my part, having global play counts, and a single place to edit library views more than offsets the downsides for me.