Let's not confuse channel and frequency. An 8 channel radio controls 8 different functions on a single aircraft. ie: elevator,rudder, throttle,ailerons,flaps,gear,and a couple lest over for things like brakes and bomb drops. A radio with a 6 model memory flies six seperate aircraft on the same frequency. You don't need to adjust between frequency with this setup since you can only fly one aircraft at a time.
Adjustably frequency radios come into play when you are at a busy field with a need to fly. Contest, racing, and the such.
They have their place but really not needed by the average pilot. At least until they become the norm.