I beg to differ. The advantage of central plugs is better control of the flame front during combustion.

Typically (during race use) the stock front head of the 21/28 suffers detonation on the front edge. To control this we'd run richer than optimal jetting, lower comp, less advance etc etc, all the things you don't want to do. Throw on a rear head the front pot and the problem is much reduced.