Bec voltages drop as the power supply voltage increases, so does it's ability to support loads, most onboard 3 amp bec's will give you 3 amps at around 6 cell nicad if i'm not mistaken. I have never needed to run a parkbec and my rx's allmost allways seem fine.

I recently had some trouble though, i used some very very thin and light guage wires to extend the esc's rx leads and it caused pretty bad glitching, i guess the resistance was too much to power the radio and 4 servos. I changed them to standard servo wires and it's perfect. So extensions could be a potential source of problems.