I think you guys are missing the point. The point is not to make diamonds more common, because if that actually was the point, the probabilities would be a lot higher. Right now, there is only about 4-8% more diamond, which is barely anything (equivalent to about 1 extra ore per 5 chunks). I don't see why you are saying this is OP.
The main reason for this suggestion is to give a reason players to mine coal. If you see a coal vein, then you know that there could be a diamond hidden in it, so you want to mine the coal because you want to find diamond.
And it would take the developers about 5-10 minutes to code in. Not even exaggerating with that one.