http://wowwiki.wikia.com/wiki/Patch_2.3.0
"There are now options for displaying health and mana values on your health/mana bars for you/your party/your target and to display that information either as percentages or as numbers."
This is what caused the confusion most likely. With that change the support for displaying HP and mana values was added only for your own and party members (both on the side and when you target a party member). It does not include creatures or players of the opposing faction, just the players of your party/you.
As some proof (without having to link videos of the time and too many sources)
http://wowwiki.wikia.com/wiki/Patch_...ented_changes)
"Health values appear on targets; mobs, boss, NPC, or player (regardless of group or faction)."
Proper support for displaying hp/mana values for both players and creatures in the game was added only in wrath of the lich king. I hope this cleared up the misunderstanding