In the 80's I built a Goldberg Chipmunk with an OS 90 4-stroke that had a single standard aileron servo, and you couldn't fly a plane in a more punishing way that I did.

Now, a 40-size airplane is set up for two servos.

The answer is simple, a few years ago, going to two servos was the rage and every manufacturer jumped on the bandwagon.

Last year's "rage" was lipos and brushless motors.

This year it's 2.4GHz systems

Every few years manufacturers need to have something to make their products "New and Improved". I'm not being cynical, that's just life in the world of sales. Sometimes, something truly unique and innovative comes along (Like brushless motors and 2.4gig systems). But when it doesn't, they have to rely on creative marketing to make you THINK you can't live without whatever the "flavor of the month" is.