The parallel hybrids can already do that because of the CVT.
In practice they also allow the engine to run at higher speeds under heavy acceleration because the peak efficiency RPM and the peak power RPM are different and the assumption is that if the driver is stomping on the accelerator they want to resolve the power/efficiency trade off in favor of power right now, which is another thing the series hybrids can't do.
In practice they also allow the engine to run at higher speeds under heavy acceleration because the peak efficiency RPM and the peak power RPM are different and the assumption is that if the driver is stomping on the accelerator they want to resolve the power/efficiency trade off in favor of power right now, which is another thing the series hybrids can't do.