There are two reasons I'd like a dual throttle in my EZ.
The first is to give give my wife and friends some basic flight instruction for educational/fun purposes. Since the EZ needs little/no rudder for shallow turns, the stick and throttle seem enough.
The second is to give the GIB a chance to safely land in case the pilot is incapacitated (bird strike, medical). It seems the GIB would stand a better chance with a throttle rather than waiting for the plane to run out of gas.
Landing from the back seat would be a challenge, especially for a novice. Why add the additional task of making that a dead-stick landing?
So, in the second case, the purpose of the throttle would be to help make an emergency landing by the GIB more survivable.