Simply untrue. I wish this annoying statement would just stop being claimed. The ridiculousness of this position comes simply from an egocentric view of the game which does not take into account healer abilities and their +% to healing as well as their maximum healing amounts when in the right gear.
Concisely put it is less effective to heal a Paladin tank as their lower HP results in more overhealing. Warriors on the other hand benefit from the maximum of each classes increased healing ability (their own, the healers, and any additional auras) which then result in far great amounts healed with far less over healing and agro strain on the healers.
Again, perfect example of flawed theory crafting gone awry in practicality.