I leveled through questing in pandaria as a disco priest and it worked completely fine. Get glyph of holy fire and smite, bring mana drinks instead of health foods, and make pandaria sparkle. I preferred it to shadow for questing actually just because of the survivability.
While some people don't mind leveling in a heal spec (and it's certainly a lot easier nowadays), it still can be a frustrating endeavour that not everyone is interested in. I can't see Blizzard making a class that would not only have niche appeal but be prone to leveling and solo frustrations.
50% more healing specs doesn't mean 50% more healers
If you don't want to heal you're not going to start because they added a new class
50% more healing specs should in theory mean more healers. Not necesarily as main characters, but as alts, surely. Just because we already had protection warriors and whatnot to tank didn't mean that the introduction of Death Knights didn't produce more tanks