Why wouldn't they want him to know about 6v batteries? Servo companies getting a kickback from the 4.8v manufacturers or something?

OP- if you are going to buy a charger that can charge and cycle 6v packs, it's good to go ahead and have them. If your voltage is marginal you still have some slush until you land and check it, and you get faster servo movement. Greater torque is also a benefit, but you won't notice it on the beginner planes at all.