Looking for some opinions here.

We've got three physical locations and site-to-site VPN between all three. Currently we've got Windows domain controllers at each location, with roughly 50 clients at each. The domains are currently separate, and we're looking at integrating the three sites.

Email (Exchange) will be located at the primary site, and RPD is already being used at the secondary branches to hit the app servers also located at the primary site. The bulk of the local user load at the other two sites is just file sharing.

What would the main benefits and drawbacks be of replacing the local domain controllers with NAS devices, and only keeping the domain controller(s) at the primary site? (assuming upgrades are coming regardless) Under what circumstances would you choose one setup over the other?