We add the 50 for all NPA's so that MDA is treated as a DA.The approach is flown as a stabilised nominal 3 degree slope.We do not add 50 for circling approaches as a level segment to a mandatory missed approach point may be needed to visualy find the field.
To the level sector advocates I would recomend the following.Go and stand in a field two miles from the threshhold ,maybe in a valley with trees,add a gusty wind and low cloudbase.Now imagine a 747 with 300 people on board dragging overhead at 500 or so feet ,gear down,engines howling,IMC,following a wobbly little NDB needle the size of matchstick which may or may not be pointing roughly towards the field if there is no Cb activity!!.Now ask yourself if that really is a sensible method of operating a large commercial jet.