Just laying out the facts without being a douche here.
First: This change is unlikely to go live.
Second: Correcting your math.
The
[Sacred Shield] tooltip doesn't show it but this ability uses a spell coefficient equal to 75% of the paladin's spell power. Meaning your Sacred Shield will absorb 500 damage plus 75% of your spell power.
Assume you are an average holy pally with 2200 SP. Your Sacred Shield will absorb 500 + [2200 x .75]= 2150 damage every 30 seconds without any talent buffs. Using 12% of your base mana to mitigate 2150 damage/30 seconds is not really worth it. However, Sacred Shield also grants a Heal over Time affect from your FoL for 50% of its heal. IMO, that is worth it.
It appears that you are a prot pally. In decent gear you should have around 1000 spell power when buffed. Without the talent, 500 + [1000 x.75]= 1250 damage absorbed. Apply the talent for 3000 damage absorbed over 60 seconds.
Finally, I am not trying to justify taking this talent or prove Sacred Shield's worth, but rather I am correcting your math and assumptions of how it works.