By connecting the motor pulley directly to the microfocusing knob, we are trying to make attaching the autofocuser as seamless as possible. This method is quite unintrusive, as it does not require focuser axis knob dissasembly, attaching directly to the axis shaft via coupler, etc. In our experience, belt slipping is a non-issue due to little force required. If the belt tension is appropriate, autofocusing works like a charm. This approach has following pros:
compact mounting balance-wise in comparison to shaft mounting, which protrudes away from the telescope tube.
driving microfocusing knob does not require a lot of force. This means that autofocuser motor can be quite small and does not require gearbox.
if you racked the focuser in/out and keep on moving the motor by mistake, it is not strong enough to damage any parts of the focuser shaft (motor starts skipping steps). With coupler connected to the focuser axis directly this could not be the case. Usually stronger motor or even a gearbox is involved to drive the shaft directly. This could result in damage to the gearbox, coupler or in worse case focuser itself.