I believe this is hidden from us to prevent the player base that is categorize as "elitist" or "perfectionist" from alienating other players from not being geared (or what they consider to be "geared") enough to break through the numbered HP.

I'm sure you'll agree that in other MMO's the amount HP on bosses, HAS been taken into heavy consideration when choosing which type of jobs to bring to the fight.

Perhaps it's better for the community that this numerical value be kept hidden to prevent further "Exaggerated Planning" when selecting party members. :/