The ONLY way WHM adds RDPS to their group is through their own damage. Both SCH and AST provide RDPS in the form of their buffs while also being able to contribute damage themselves.

It would be unfair for the other healers to have those buffs while ALSO having the utility of a stun too, especially when AST is so much more friendly when it comes to weaving abilities meanwhile WHM has to clip GCDs just to heal. While SCH might not provide the stun or party buffs that AST does, it at LEAST has a buff for bosses. A small increase for dungeons but it's better than literally nothing.

In fact I wouldn't be against SCH having even higher potency on AoW to make up for the fact that their aoe has no stun and they don't provide any sort of aoe offensive buff themselves. But to give AST anymore would be ridiculous.