Well I probably have been over thinking this but here goes!
Most builds for a quad have four ESC's connected to the output rail of the APM. If you are using ESC's that have a BEC then there will be a conflict between each BEC on the rail. Explaination: Nothing is perfect, if the ESC's are labled A, B, C and D. If ESC A is supplying 5.01 volts and EXC B is only putting out 4.98 there will be a difference on the buss of .03 volts.
One of two things can happen. Good: ESC B will sense the higher voltage and shut down and only kick back in if A fails. BAD: ESC will try to sink the excess power, ESC A will try to compendate. Very messy! Could make magic smoke.
What say you?
Replies
Already discussed this in some other threads, the power distro board only connects one of the BECs to power the APM and radio, the other ones (5V pins) are not connected to anything. This is why it's important to use a power distro board and not roll your own.