remove red wire?

[BrianG]

You'll drain the batteries over time because there is some "leakage" current.

But, I think you meant "What happens if you leave the BEC switched on and still use a reciever pack?"? Well, that depends on the regulator. Since a receiver pack is typically at higher voltage than a BEC, the receiver pack should take the load. Once the receiver pack voltage falls below the BEC voltage, the BEC takes over (in fact, the BEC would be charging the Rx pack at this point). This assumes that the regulator is designed how I think it is. If it's not, hooking a Rx pack and the BEC could potentially damage the BEC.