You're assuming that those lost rounds aren't taken into account when getting the right HP to Damage to AC to Attack Bonus ratios. But they are. In a hypothetical world, a class that has 1/2 the HP of a normal character would last half as long, but each round they get would accomplish twice as much.
No, they're not. It is virtually impossible for the designers of this RPG to be able to accurately determine how often PCs will fall unconscious, how many rounds it would take for them to maybe or maybe not be brought back from consciousness by any possible amount of clerics in a party, and how much damage would be lost or not soaked from those man-rounds of unconsciousness because of the variance of which classes are still on their feet and which ones are or are not engaged in melee with the monsters. Because much of that is also based upon how individual tables play the game.
So any attempt at reducing the power of the cleric in direct proportion to the amount of resources they end up saving the party so that you can keep the cleric "balanced" with the other three classes is going to fail. There are TOO MANY VARIABLES.
It is impossible to balance every class against every other class in every possible party configuration and every possible game situation. There will ALWAYS be situations where one class (or having more than one of a class) is the OPTIMAL situation to have for a particular encounter. And thus... resources will not be lost as fast, and thus that particular party will get to push on to another encounter that another party wouldn't.
And there's nothing wrong with that!
So long as 98% of all parties remain comparable to each other and can run between like 3 to 5 encounters, or 4 to 6 encounters... that's good enough.