I accept your point about driving bans imposed when the person banned will be in prison for the entire duration of the ban. However, one thing to remember is that when someone is sentenced to, say, three years in custody they will certainly not serve more than 18 months and probably a lot less than that. A disqualification for the part of their sentence which they do not serve in prison makes sense.
As far as your other point goes, Chris (Buenchico) has made this clear. Driving without a licence, or when too young to have one, or not in accordance with the one you hold is considered as really an �administrative� offence. Driving whilst specifically disqualified by a court is open defiance of a court order and is viewed much more seriously. It is one of the very few driving offences for which one can be sent to prison.