I have a separate switch that contols power to my dual SkyView system. Normally I just leave this in the "ON" position, but just last week I had an alternator failure in flight, and used the switch to turn off the ship power to the SkyView system, which in turn caused the SkyViews to revert to their respective backup batteries. The negative draw in the ship battery went from -9amps to -3amps. In that mode I could have flown quite a while and still had all my instruments and radios. So while you don't "need" a switch, it is nice to have the choice of internal or external power.
Best Regards,
Mel Jordan