Numbers are too "technical" looking; in the Minecraft world itself (thus not in the GUI's) you see never any number at all, not even on the clock and the compass. Only if you press F3, you will get numbers on your screen, otherwise not.
We have already those sparkles when you critical hit a mob. I don't see why they could not just use those instead, with less sparkles by less damage.
well, you wouldn't see a HUD under your vision, and those particles disappear too quickly. AND those are meant for critical hitting a mob. but, seeing you can't use much of anything to tell how much damage you did, hearts look like a good idea now. 3 hearts dealt, 3 hearts fallen.