The reason they never incorporated ADB was it hammers a fairly reasonable current down the wire to power the devices (like USB) and it'd have drained the battery in fairly short order.
This is also why the ADB ports on Macs are thermally fused, if the wire drops out while the machine is running it can short and it was known to kill the port on the first ADB Macs (yes, Mac II owners beware!). Apple subsequently fitted a thermal fuse to the power circuit on the ADB ports to prevent this happening. I've had Macs lose the ADB port for a few minutes.
I believe the power-on signal on a Mac ADB keyboard also relies on grounding the power to the keyboard back to the computer down a certain line, although I could be wrong.