Holy, holy, resto, resto; they're the four healing specs. They're also the worst leveling specs. Even worse than slow leveling, they might create bad healers.
While leveling up, any instance can be healed by any spec, assuming decent gear. It's not always easy, but it's possible. Being a healing spec would make the healer more effective, so how can that be bad? It teaches a false sense of security and power.
When someone makes a bad pull in a lowbie instance and have a shadow priest healing, they might die. Do the same with a holy priest and they're more likely to live. The more effective healer shelters people from the consequences of their actions. No deaths, no learning.
On the healer's side, it's about as bad. They are more efficient and heal for more. There is very little challenge. What will happen when they get to heroics or raids where being specced for healing isn't just an advantage that trivializes the content, but a requirement to even stand a chance? Healing while leveling is easy enough already, doing it with a healing spec is overkill.
Finally there's just the fact that healing specs kill slower, so they're inferior for soloing. This makes leveling take longer and it can become frustrating. That results in more alts stuck at low levels with the player turned off to the class.