There's still an advantage to having a hybrid system onboard even at highway speeds. The continuously-variable ratio of the hybrid transmission allows increased freedom of the ECU to select {instantaneous gasoline engine RPM, instantaneous gasoline engine torque}, which permits operation in more favorable parts of the BSFC map more often.
Furthermore, the battery/motor-generators allow the ECU to temporarily run the gasoline engine at lower or higher output (without loss of traction power and without waste) than the power required at the wheels, further expanding the area of the BSFC map that the gasoline engine can operate at.
It is true that mechanical-electrical-mechanical conversion is more lossy than an all-mechanical system, but topologies such as the Toyota hybrid system allow for significant power transfer without electrical conversion; and spending significantly more time in more efficient regions of the BSFC map makes up for the conversion losses incurred.
Also, hybrids tend to have electrically powered coolant pumps and electrically powered air conditioning compressors, allowing for more aggressive fuel cut off (on a downhill section of the highway, say).