Stump,

The reason for running BEC has nothing to do with battery capacity or size or characteristics. BEC is run because most ESC's that run 4S or higher battery configurations cannot supply power to the motor and the receiver without generating excessive heat or shutting down. The amount of heat and shutdown is realative to the number and type of servos used, along with the model of ESC. So in answer to your question, should you run BEC on a 4S or larger power setup? The answer is yes.

Carl