Certainly for something like a mill which might be used for tapping, then reverse is essential. Must admit, though, only time I've needed reverse on my mill was when I made a custom T-slot cutter and realised just before I turned it on that I had made the teeth left-handed... Had to temporarily rewire the motor in those pre-VFD days. Ho hum...

There are some mills with a back-geared spindle as well, where you run the motor in reverse in the low-speed range. But for reversing, you do need two separate outputs from the BOB with the capability of both being "off" - a single change-over contact and relying on the "speed" signal to stop the spindle is not enough.