99.9% of the time, tools which work for communicating over the internet work very well for communicating locally. It's not worthwhile for most people to devote the time to building, maintaining, or learning a second tool for the LAN side for that 0.1% of the time when your internet connection is down.
True, but they are rarely set up properly. Also, when they are set up they are often only used for things that are thought of as 'local tasks' such as a media store or printer server and not for replication of the online system that you have for some insane reason decided should be hosting your accounts or whatever.
