Not sure about the Rustler, but on the T Maxx there is a reverse kit that would make use of the third channel. I can't remember offhand the name of it, but Traxxas does make a nitro truck with reverse.
My guess is they can get the radios cheaper by making them all three channel, rather than making some 2 channel, and some 3 channel. I do have a T Maxx, and have been wondering what else I might add to it to make use of the third channel. Lights, or anything else.
If anyone has any ideas, feel free to drop them here for me.
They have even stepped it up with the new rock crawler, which uses a four channel radio. The third channel is to switch between high and low gears, while the fourth channel is used to lock in the diffs. Pretty sweet setup, though I haven't had a chance to see it in action other than the factory videos.