[BrianG]

The ESC uses a BEC no matter if you use it for your servos/Rx or not. If you don't use it (removing the red wire to the Rx), the internal BEC simply provides power to it's internal controller circuits. Those circuits hardly draw any amps so a linear BEC is OK. It's when the same BEC has to supply power to other devices that heat becomes a problem. For a linear regulator, power dissipation is calculated by (v_batt-6)*load_current. So if you are using 14.4v batteries and your servos draw 4A total, that's almost 34 watts the regulator has to dissipate on the same heatsink the FETs use. The switch simply cuts power either to (or from) the BEC to the internal circuits thereby shutting off the controller.