First I want to state that I am also not an electrical engineer but my job does involve working with electrical systems, wiring and large electric motors so this subject is not foreign to me.

The problem is not with the receiver, it's with the power distribution system as a whole. The receiver provides the bus that all of the components connect through. If you consider that a servo draws a given amount of current under a given load then X number of servos multiply that load, across the receivers bus. All those little motors in your servos draw a given amount of amperage at a certain voltage, allow that voltage to drop and the amp draw goes up accordingly. When the amp draw rises so does the temperature of ALL of the components in the system, especially any point of connection. As temp goes up on electrical wiring systems so does the internal resistance which compounds the problem even further as it's also reduces the voltage and causes the amp draw to rise more. It's a vicious circle that can add up quickly and have undesirable results. This is the whole reason it's highly desirable to use HD extensions with gold plated ends and heavier wire in the first place, to keep the voltage constant as much as possible, preventing an unusually high amp draw for the system as a whole.

While two batteries operating through 2 separate switches are great for redundancy in the event of a switch failure, they are also provide double the capacity on the bus, an added and highly desirable benefit.

Also something to bear in mind is that Li-ion batteries that have a high voltage rating are regulated down will hold the voltage at a constant level much better than Ni-cad or Nimh. Remember that the nickel batteries start out at a lower voltage and go down from there so as your batteries deplete the amp draw rises accordingly. On smaller planes this is a non issue but on larger models it can actually be felt in the control response of the model by a highly experienced pilot.

It's really a no brainer that once you get over 30% scale that Li-ion becomes the battery of choice for the reasons presented above, on smaller models with fewer servos it's not as critical to the performance of the system.