Battery disconnect switches are pretty standard on boats and I often wondered why not on RV's. In addition to disconnecting all the loads when you store the rig, as noted above it is a safety feature. Should you have an electrical fire in the 12 VDC system, it is nice to have a single place to quickly disconnect all sources of power. Perhaps a little overkill, but then again I worked as an Instrumentation & Controls Design Engineer in a nuclear power plant for over 30 years and if my rig didn't have one, I'd install one.
Lou
05 Travel Supreme Envoy