With a BEC providing your main battery is well charged, you should always have a good, healthy 5 volt feed to your radio. A separate battery presents an additional failing load IMO.
The problem with BEC's on some ESC's is they're minimally specced, or poorly heatsinked, and are often poorly documented on what they can really deliver in terms of performance. That's why I stick with, and recommend certain ESC's especially for use in submarines which tend to be a bit more demanding than most model boats.