I got two words for ya, Jim: Field charger

On up to 10-cell ships, the built-in BEC is immesurably handy in that you can save a bunch of weight by not having to haul around a receiver battery, and you can save a bunch of headaches checking and charging a dedicated receiver pack.

Don't let 'em make you think that BECs are prone to fail, or that ESC don't come with cut-off circuits. BECs are usually the last things to fail in an ESC; I can't count the number of times I've read about someone smoking an ESC and saying, "but the servos still work."