The reason why there should only be one BEC feeding the RX/servo part of the system is that there is no certainty that all 5 volt BECs give exactly the same, and at even a few microvolts difference, they start arguing. A BEC equipped ESC with the red wire disconnected is still using its internal BEC to work its own control circuit. If an ESC that doesn't have a BEC has its red wire removed, it will stop working. Some ESCs have their "power" switch in the feed to the BEC, if this is "off", and no red wire, the control part cannot work.