@Christof: Of course you're right. For hobbyists a simple capacity measuring is the easiest and cheapest way to get a comparatively good result. At least much better than to rely on voltage monitoring.
@Max: Emcotec produces reliable battery switches since years: . They still work to get a high current quad switch for our copters. The problem is that it must be light and small but be able to handle high currents. Not everyone wants or needs to fly with 4 batteries so it should work with two as well which means that in an emergency case one battery provides the total current.
Btw: What I will enjoy in our copters is the magnetic switch to turn main power on and off. You don't have any flash when connecting the battery and you don't need to care about shaking the copter during the time you fix the batteries which is usually also the initialization time of the AQ. Instead you just turn the copter on without even having to touch it.