There are people who use all 9 channels and then some. But usually, what you are looking for in the higher channel trasnmitters, is the higher level programming.
My Futaba 9CH just had channel 6 used for the first time with the purchase of a T-Rex heli. I could have used a 6 channel transmitter, right? But I purchased the 9 channel becuase it does all sorts of fancy things that make my life easier.
High and low end point adjustment.
5 point revo mix.
Multiple channel ailerons. (there are gliders that have 6 channels in the wing alone!)
Many, many assignable switches and dials.

A few of us fly foam wings at the inflatable sports dome. 24" Zagi style wings with CD-Rom motors that really move fast for an indoor venue. I can JUST squeeze out a roll before it is time to bank and yank. One of the pilots that had a much faster wing could get 3 rolls. In order to have that much control throw, I would not be able to fly the plane! He explained that he used one of the switches. Press the switch and the ailerons go full travel left. So he has small throws for flying and full throws for snap rolls.