I might be misunderstanding something here, so feel free to correct me. But the problem with your original circuit was that it went from -15V to +3.2V, right? And it needed to go from -15V to 0V. Well, a diode would accomplish that, although it would drop ~0.5V when conducting, but that shouldn't matter too much.If you used a diode to remove the bias, you'd have to ensure that there was a constant current flow through it, or the voltage wouldn't be very stable. It would add to the parts count, too.