The weird thing about hp is that buying it rather than armor/mr produces different effects against the different damage carries. Against AD carries stacking hp is going to be more helpful to them than stacking armor because they can lifesteal more off you. They'll be able to get back quite a bit of their health and undo most of your burst. An AD carry doesn't want to buy armor pen, he needs to when the opponent buys armor.
AP carries are kinda meant to do a lot of up front damage, but have lower dps than an AD carry overall. So against AP carries, buying hp with a little mr is going to be much more helpful thank buying a ton of mr. The (1000) hp + little mr combo is basically going to negate a full combo from any mage around mid game, which is problematic since they want to be useful too. But now instead of having 100 useless mr against an ad carry, bruiser's will have extra hp, which makes whatever armor they have more useful. So building lots of armor, hp, and a little mr is going to be really good defensively and make AP carries sort of useless.
With the advent of blackfire torch, I think mages can actually have a midgame answer to stacking hp. So instead of countering mages with warmogs and merc treads, tanks/bruisers will need to buy some mr or risk the bft passive burning them to death.
tl;dr: AD carries will be happy that enemy bruisers spent 3k on "not-armor/damage" and AP carries will be happy that the bft they're always building anyway works extra well. Warmog's has few reasons (if any) to be kept out of Dominion.