Believe it or not I have two aircraft (a Goldberg Hot Stik .40 and a Great Planes Ultimate 1.60) that use all eight plugs in the receiver and multiple mixes and assigned switches on the radio. With the Ultimate I even had to bring the batteries into the circuit with a "Y" connector into the 8th channel port! Neither of these have any fancy features like retracts, smoke, bombs, etc. I do have an optical cable engine kill for the gasoline w/electronic ignition that takes the retract port on my receiver. Otherwise, just the basic flight controls. When I bought the Futaba 9C four years ago I had no idea how soon I'd be using as many features as I now do. The Goldberg has four aileron servos and by keeping them independant I can use them as ailerons, flaps, spoilers, flaperons, spoilerons (C.R.O.W.) and do all kinds of interesting mixes.
WHen you start to get dual rudder servos and split elevator halves having the versatility in the radio saves cost and weight by not needing to add seperate servo combiners and balancers to the aircraft.

Soooo. My vote would be for an eight channel for full versatility.