RE: Nexstar engine and servo's question
The reason for the 5-6 servos is that the latest fad is to use 2 servos on the Ailerons (1 for each side) for a 40 size plane, this really isn't necessary.
Other than that, extra servos can be used for retractable landing gear, Flaps, Smoke, and just about anything you can think of.
BTW, don't be frightened by it, a standard servo can be picked up for about 12 bucks.