Battery assistance is not just for low speed around town. At high speed the electric motor continually cuts in and out assisting the engine whenever it detects any high demand for power. This is undetectable unless you look at the power display. This is primarily to relieve engine load and improve MPG, it also boosts power when accelerating hard.
When I owned an LS 600h MPG never fell below 34 MPG, mainly high speed cruising. Not sure if the power animation shows this on the LS, it does on the RX.