Yes most people don't seem to realize the hybrid system tends to just shut down at highway speeds. The reason a hybrid car has good highway mileage is because it has skinny tries and good aerodynamics, if you ripped out the hybrid gear its would get even better highway mileage due to less weight.

There is another aspect specifically of gas hybrids they typically use Atkinson cycle engines to increase efficiency but have reduced torque. This makes it ideal for a hybrid setup as the electric motor can make up for the lack of torque off the line.

Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than a Atkinson cycle gas and has even better torque than a Otto gas but they weigh more and cost more, so it makes little sense to add more weight and cost to a diesel drivetrain that is already very efficient and has plenty of torque. City buses are the exception due to the extreme stop and go low speed duty cycle that can really take advantage of a hybrid drivetrain.