I have looked into this and what I gather is the BEC will offer a steady voltage to your servo at 7.4v for example whether it has to step that up or down a little bit.

The downside is that if you are running a lot of gear off the receiver, it will draw more amps and could exceed what you can feed everything at once, so that transponder and those two fans plus a high torque servo might all add up to the be a bit demanding. Sometimes you can steady that out with a glitch buster cap, maybe you have enough coming from the ESC to not be an issue, or maybe you need an external BEC.

From the support folks at Savox I am told that they like to see a BEC used on electric setups. But that direct lipo will work. The reason they like the BEC in play is the steady voltage. The claim from them is that some batteries will dip a bit in voltage as they are hit with a lot of demand from the motor. An ESC has a bank of capacitors in it to smooth that out, but I am sure that the dip will vary greatly based on real life - battery used / motor used / ESC used / vehicle setup / driving conditions.

My advice is to step through it until you have reliable results:

1 - Try your built n BEC.
2 - Then try the built in BEC with a glitch buster.
3 - Then it is time to decide if you need an external BEC or want to wire in direct 2S lipo.

You just quit the list when you have your simplest solution.