By the way, Airbus don't allow setting the MDA in the ALT window.
That is the crux of the matter. If doing a CDFA ie straight-in runway approach it is logical to set the Missed Approach Altitude when descending on the approach, as an immediate Missed Approach will be conducted if not Visual at the (derived) DA of MDA+50.
But on a Circle, where, as pointed out by Buzz, there is no need (in fact it is undesirable) to do a Go Around if not Visual when first reaching the MDA, ideally the ALT SEL would be set to the MDA and the aeroplane allowed to level off automatically. The Missed Approach altitude is the preset in the ALT SEL. Circling approaches in my "Boeing" are done just that way.
If you are
not allowed to set the MDA in the ALT SEL, then you're stuck. 50ft above MDA may not be enough when pushing the button, as described by Vilas.