Well, I can't speak for anybody else but I doubt ANY design from scratch would be correct first time. If you don't use a microcontroller any design modifications aren't going to be quick or simple, where as if you do use a micro then you whip it out, reprogram it, and then put it back in and can see if the modifications work. What you have got to bear in mind is that the basic discrete electronics systems (psu, motor drive circuitry, sensors etc) must work in the first place (as they must in a robot without a micro) all that the micro is, is the 'brains', the control system. I suppose one disadvantge is that you have to be able to program the micro in the first place and its probably more expensive.
This is a good question, consider posting it on sci.electronics.basics newsgroup for more information.

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