I think Ghostcrawler is right about the mana regeneration being too low in low gear. (here)
I have a priest alt with entry gear for 378 5mans and LFR, and it often feels like what I need is not pure spell power or other such "DPS"(output) stats but pure regeneration. I noticed the most "prudent" players that want to keep their mana very high for practice, keep the tanks in 5mans of 378 on purpose low so they can heal them when it's only absolutely necessary (when the healer is entry level). Well, one could of course claim that max utilization of regeneration spells might improve the picture more than what I describe but it comes a point that it feels like this max utilization and perfection is probably too much for the needs of some common non-Heroic (raiding) healing.
Then again I'm thinking, this is a bit dangerous to create a situation where DPS gear is more valuable so, yeah,
The right solution might be huge mana pools (with a bonus multiplier), with low regeration and big bonuses on healing spells when the spec is healing so that spirit is valuable, mana doesn't drop very rapidly and "DPS" stats do not easily beat it.
So yeah, they might have to return to spirit as primary.
What do you think?