The issue with "Pure" vs "Hybrid" characters is a never changing issue of balance.

Either the content is balanced for a "pure" healer, pushing the other healers into a secondary role in which you'll always see WHM +AST or SCH, or it's balanced for hybrids, pushing WHM out of the picture entirely.

Considering they want to keep Paladin a "pure tank" and WHM a "pure healer" I get the feeling that SE is going to balance around that and push DRK/WAR into offtank roles and AST/SCH into off healer roles.

At that point, it's a question of whether the AST or SCH ends up as the benchwarmer. If they're fairly well balanced against each other, it could be an Either/or situation, but considering how people want "optimal" even if it's a 1% difference, we'll see.

Even so, We don't know what the new skills really do or how things will be balanced so it's all speculation. For all we know, WHM could hit so hard that it displaces SCH as the offhealer of choice. Not likely, but I've seen stranger.