I think the reason the first patch for this feature was rejected was because it was basically a bmesh operator in the modifier. See this topic: https://developer.blender.org/T41748
Also see this topic: Hurdles to implementing Merge modifier?
The great thing about OpenMeshEffects is that anyone can write any effect for mesh data- even if it doesn’t meet Blender’s strict code review standards. That way, anyone can hack together a quick solution, share it, etc. Of course they can also make a really nice solution, or polish the quick hack into a nice solution. The point is that it enables a more lenient development process because it isn’t managed by someone else, while still providing an easy way of sharing the modifiers you create.