Its not just the cost
I'm basically want to make controlling outside world as easy and cheap as possible from the RPi.
So at the moment, the only external component I'm using for both DC and stepper motors, is a ULN2003 at £0.37
As well as being cheap, its easy to explain its function in beginners terms (buffers the RPi from ill effects from motors) so I love it
I do realise that once an external MCU is added, controlling motors/servos/analogue input is very easy to achieve (for us lot and I could make the interface invisible to pupils/teachers) but I feel that that's a step too far in moving away from the the RPi can do itself and would be a case of - just plug this little circuit in and watch what it can do.
I just need to wait a little bit more time for the servo wizards to give us the cake with the extra icing on the top