Does it teach anyone to be good at their role? I don't think so.
Those that are good become good because they are willing to put in the effort to learn from other sources. For others this is just a game and not worth that effort.
Where the game does a disservice to healers is with ilvl scaling and mitigation abilities. More HP means incoming damage takes a smaller percentage of HP and so lessens potential urgency to heal. Mitigation abilities on top of that means HP is so small the need for healing is infrequent. When the amount is reduced to the point that a player does not need healing at all to survive an encounter, how can a healer learn to heal?
Someone else mentions job design as the problem. The job team could come up with the most amazing job design for a healer that's the envy of all other MMO developers and it will mean nothing if the encounter design team doesn't put that design to good use. If healers feel like they're nothing more than green DPS, it's not the job design at fault. It's the encounter design.