A guy who lets a guy flatten him out in half guard and elbow him doesn't know how to play half guard. Rule 1) underhook, rule 2) don't get flattened out.
Closed guard can be effective in MMA..frankly, any guard you use in a no-gi match can be used in MMA if you remember the basic rules:
1) control posture
2) control grips (including underhooks and overhooks)
3) create angles
This is the battle that guys in MMA are losing too much while on the bottom..they are getting flattened out, letting their opponent comfortably base and not playing with their stability, and not controlling grips effectively, and letting the top guy stay squared up on them. It seems like in the guard, the only thoughts are "don't get hit" "get up" or "submission".