(double post removed)
I think the quite enlightening conclusion that comes out of this discussion is that D&D rules actually give rather a good basis for magical medieval feudal warfare, with small bands of superbly equipped knights & heroes fighting each other, and peasant levies regarded as pretty much useless, there only to cheer on the glorious heroes.
A problem with appraising the impact of D&D magic is that people have traditionally tended to assume the existence of professional armies, which in the real world existed only from the 18th century onwards, and in particular the 'mass citizen army' - huge numbers of (1st level, in D&D terms) soldiers, which in the real world date only from ca 1800 around the time of the French revolution. This style of army dominated warfare for 200 years, but it's very arguable that we are now returning to a more medieval paradigm, with the emphasis on small groups of 'knights' - whether those are special forces or Apache attack helicopters, and the mass citizen armies disbanded (eg the UK now has only 100,000 men in its army) or relegated to impotence - like Iraq in the last Gulf war, or the effectively useless conscript armies of mainland Europe.