Since when did it become accepted wisdom that the no7 position had to be a frontline batsman? In a traditional line-up with no all-rounders, no7 was the keeper, expected to average in the 30s. England have the bonus of an all-rounder - Matt Prior - who averages 40 in tests and can bat 6.
Broad averages 30 with the bat and is clearly on the verge of doing a more than competent job at no7 - and he won't learn and improve unless he bats there. You won't learn anything about building proper innings at 8.
Contrary to what the commentators are going on about, England do not have the same balance as South Africa - England have a frontline batsman/keeper, South Africa have a keeper.