They do it so that you can get the higher power if you want it. You don't have to go up to 7.2v if you don't need the torque and speed, but if you do you have it. So those servos allow for a lightweight and compact servo installation with tons of power. The cost is having to use regulators. But I can't imagine a 1/4 scale plane that requires 350 oz in servos. My 50cc Extra 300 doesn't even need that, not even on the rudder. So a 6v setup should be fine. There are 2 routes you can go with that. The first is to choose a more robust main battery to run your electronics and then use a BEC (an absolute must to prevent interference) to run the ignition, and the second is to use separate batteries for both. The advantage for the first is simplicity in charging and maybe a tiny weight savings. The advantage to the second is that a failure in your ignition, BEC, or wiring for the ignition won't also shut down your control system. I'm a fan of keeping them separate personally, but both ways work.