I've often wondered about that line of reasoning for a separate Rx pack:
1. If you're using, say, 7.2v packs or higher, a "low" battery will still have plenty of juice to supply 5v and a few milliamps to the Rx;
2. And if the main pack is down that low, what good is radio control? You won't have anything to command!

I always just use the BEC; it's a good thing.

Two 700's on one of those ESCs? Good luck. Most ESCs are not capable of delivering the advertised current. Buy one more now for a total of two, vs. toasting one and buying a total of three!

Pat M