That's simply not true... while most ESCs do have a BEC these days, there are still some that do not...
http://www2.towerhobbies.com/cgi-bin...?&I=LXTF90&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXTF93&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXTF92&P=7

Still, I believe the easiest/cheapest solution is the one I described in my previous post. For a non BEC ESC... if the ESC control circuitry gets it power from the RX, then you won't have any drain on your ESC battery when the RXis off, but if the ESC controller gets it's power from its own battery terminals, you still need another switch on the ESC battery connection...