Interestingly, there is a difference between HP and armor bonus when it comes to healing: the armor doesn't reduce healing as it does taking damage. That would make an armor bonus stronger than a HP one. When Ptolemies had this cavalry hero that gave extra 40% HP to pikemen in his aura, you could heal the pikemen faster by moving them out of the aura.
I remember playing some games from the times before the exponential armor system, and there seemed to be a problem where if a unit or building had more armor points than you had damage points, you wouldn't be able to cause damage to it anymore. And so there was a hard-coded minimum of 1 HP damage (one other detail is that nowadays the HP uses the fractional part, even if it's not shown). That could mean that you were better off using a greater number of very weak units (like archers) because you had the the guarantee of at least 1 HP per arrow shot.