As of 5.0, tanks only have a couple gcd actions that contribute to tanking. Defense and enmity actions, with a couple exceptions, are handled on ogcd weaving.
However, despite that not being quite accommodated for healers (the tanks have a decent portfolio of instant gcd offensive actions), healers basically play like that to the extent that they can. We avoid gcd heals as much as possible. So it seems like a natural convergence is in order there. So it makes sense to make healers just like tanks in that regard: have a good amount of instant gcd offensive actions, and then do most of the healing via ogcd weaving.