It's more a 'flavor of everyone prefers Disc for harder content' and staying as Holy makes you a dime a dozen and interchangeable with a Druid. Disc isn't having scaling problems, mostly due to their ability to gear entirely for throughput after a certain gear levels (226'ish). That's basically how we've kept up. No mana issues without stacking int to 30k+ mana means we've avoided the stat scaling via class mechanics (and minor bug exploits :P). Also keep in mind that primarily raid shielding is optimal in two encounters in ToGC and hard modes in particular. Penance and PoM should also be on cooldown as well though, otherwise your net HPS+APS drops. That's why it does become a problem, fights with lots of consistent raid damage wind up favoring Disc on the raid. Giving them more AOE stuff would make the situation worse. Good Priests would flock to the superior spec, just as good DPS do.