Having to slug it out with low damage abilities as a healer against npc's has nothing to do with how you're supposed to play your class at max level. Healing flashpoints does.
If you have dual specs, more people are going to have a leveling spec and a healing/tank spec for instances. That means that those people learn to play their class better, and that there are more tanks and healers to go around.
Limiting people to either slow leveling as a healer or fast leveling as a dps with no ability to heal, means that a lot of people is going to choose just not healing/tanking while leveling and therefore suck at their role when they hit 50 and start doing instances.
Nostalgic people are the reason we cannot have nice things.