For the benefit of COl and 4Dogs.
The discussion is about CANPA's, constant aspect non precision approaches. It doesn't take a genius to work out that the easiest way to fly a Non precision approach is to arrive at the MDA in the correct position to be able to land on the runway. If you fly past this point at MDA in level flight you will not be in a position to make a stable approach, rather 'the duck and dive'.
The MDA thus becomes your effective 'decision altitude' because you have reached it at the ideal distance from the runway.