If the boss hits your tank for 30k, the tank needs 30k healing. It doesn't matter if it takes 2.5 seconds, or it takes 2 seconds, the tank still needs 30k healing. If the boss hits the tank again in 2.2 seconds, and you take 2.5 seconds, then your tank is eventually going to die. If the boss hits again in 2.2 seconds and it takes you 2 seconds to heal 30k, then you have "wasted" haste rating that would have been better reforged into something else, like spirit.
Just because you can go OOM in 25 seconds vs. 30 seconds doesn't matter. The same raw healing has to happen.
Just because you CAN burn your mana faster doesn't mean you SHOULD. Yeah, if you have 2 people spamming DL on the tank, one of them with haste stacked, then YES, the haste stacked paladin will OOM faster is time. However, the raw healing is the same. And say it takes 30 seconds for the haste paladin to OOM, and it takes 45 seconds for the non-haste paladin to oom, the haste paladin will still be able to regen at the same rate in those 15 seconds and will resume healing.
But they're both idiots because that's a stupid rotation.
Mana in is independent of mana out. If we want to talk about the bad aspects of haste, again, it's the opportunity cost of crit and spirit and mastery, not the fact it gives you the CHOICE to oom faster. The problem isn't haste, it's the player.