The Power safe Rx's will "powerup" as soon as you add power to the two leads, so if you leave the "soft" switch out you can use the Powerbox type switch (Sensor or Gemini) to run it and yes the only advantage is voltage regulation. Normally the Rx power goes in through the servo connections, but on PS Rx's it has no connection, this is why you can't really use the PS Rx's with a Powerbox unit.

I use the Sensor switch in most jets of BobCat/Flash/Bandit size, its been perfect, in fact the original one is still in the Prototype Classic Flash and has worked faultlessly since 2006.

If you use A123 unregulated, you are on the upper limit for many servos and for sure the servo life will be reduced. In a lab situation with smooth non binding control surfaces and perfectly set up flaps I guess servos will still last a long long time, but you will get away with less abuse at higher voltages with more heat produced.

Dw