Hitpoints are an abstraction of health anyways. If you allow players to see exact hp and exact damage, they can 'game' the system (meaning really power game and control their resources very tightly). I guess there's no reason you can't put those numbers in. Just ask yourself what kind of playstyle you are aiming for? Really, the reason for showing or not showing hitpoints/damage has more to do with player resource management than anything else.
a) The magic missile does 7 pts of damage. The goblin has 2 health left. You have 10 health "Oh, I can finish him off with my weakest, cheap spell and save some mana for the next encounter! Theres no way he can kill me in 1 hit."
or
b) Your sword slices through the goblins armor. The goblin is injured. "Crap, what do I do? Use a small spell, and save some resources, but potentially allow him to live, and maybe kill me with its next swing?? Or use a fireball spell?? Costs me more, but gives me a bigger safety margin."
(As a side note, I am kind of choosing a middle ground for my next project. In the past, I used health bars, which allows almost as much knowledge as getting the numbers (which the player also gets, if they choose to read it). I have switched over to a 'health bubble' - the color of the bubble gives a generalization of the health of the enemy, but only a vague one. Green = uninjured, yellow = somewhat hurt, red = about to die. After all, the player would be able to 'see' if the goblin was bleeding from 10 wounds, but he wouldn't know the exact physical extent of the damage. Just my $0.02)