Selfhealing is a factor of it's own. Having Healthstones is all good and fine, making people use them is another thing.
The hybrid fact is more in essense to the fact we can throw around PoMs while running, we have VE which is fairly strong as a raidhealer, depending on the fight we have either Halo (critting for 250k as an aoe heal on the raid) or Cascade, that can be used as a heal from a SP pov where needed. In addition to that we have a CD to save the tank, we can grip away people thus save them, not to mention speedshield and from a draenei pov Mana hymn.
Point being, its very nice you have your self heals, and healthstones, but the heals we provide doesnt require the other raidmember to do *anything* to gain that heal, clicking on a healthstone without it being called out seems to be a very difficult task for people in general in raids.
THERE is the hybrid tax.