You're looking at this the wrong way. The reason Grace is required for this (both talent-and-effect wise) is because it's a target you're already healing. Think of this like a tank/offtank/debuff (like Mark of th falle Champion) kind of talent.

Weakened Soul is going to either be used for big hits that it needs to be, or used on cooldown.

This is meant to put Renew on an "effective buffer zone" kind of spell (that won't become obsolete as you move into deeper content) without changing how it functions at all for Holy. If it doesn't stack, but turns into a weaker Sacred Shield or Earth Shield that can actually also bring a target's HP up between/before swings, that's still a unique effect both from Priests, and from healing classs in general.

Plus, it not stacking Divine Aegis and having it's own absorb means more room to play with how the numbers really need to work (but an actual tick should still be stronger than an absorb).