No matter how good, no hardware can consistently guess your preferences. Turning on a low power flag might ignore the anti-aliasing flag because it uses a lot of power. Or might not because the GPU can do it without much additional battery drain. There are many other features future GPUs could turn on or off to sacrifice performance or quality for the sake of battery life. I think a flag like this gives the author the ability to make some very mobile friendly content.You are not giving the hardware designers much credit and nor apparently noticing the progress made by the latest low-power high-performance GPUs for mobile devices. I'm quite sure technologies for powering off unused parts of devices will migrate from CPUs to GPUs.
There is no "need to combine both low power graphics and high power graphics in one system." There *is* a need for high-performance graphics that consumes low power. The current dual-GPU solution is a hack.