IMO, archangel was a better fit as a talent because it makes for a particular playstyle choice - not all disc priests are going to/want to dps to heal through attonement. Take live talents: currently priests can choose to talent evangelism/archangel/atonement or they can talent Twin Disciplines/Mental Agility/Train of Thought... But if it remains in MoP as a baseline disc spec ability then it will 'feel' like a waste not to use it.
I understand that Holy was at a disadvantage because it doesn't have atonement and it didn't because of the bonus to damage via Chakra: chastise. My proposal is why not just remove the holy damage component of Chakra: Chastise so that damage is similar to disc with evangelism only and revert atonement to a major glyph so that both Disc & Holy can glyph it and talent archangel to make use of this playstyle in either spec?
I recognize that it doesn't fit in the mana regen theme - so why not keep the mana component of it that it currently has live?
The reason that i'm bringing this up is because i honestly don't feel that priests need yet another damage spell in the form of PW: Solace/Mind Siphon when there are already spells you'd be better off casting ie smite healing/evangelism stacks, healing spells, shadow damage spells, dispelling etc. Why talent PW: Solace/Mind Siphon when you can talent Mindbender/FDCL and maintain your spells 'rotation' while still getting mana back?
You could even ramp up Dark Archangel to be less straightforward by making it consume shadow orbs to activate with more orbs to increase the duration. Then if an spriest player 'decides' that they'd rather the damage boost in this form rather than another Devouring Plague then they can make that choice.
Another thing, the tooltip for Atonement in disc specialisation says:
"When you deal damage with Smite, Holy Fire, and Penance, you instantly heal a nearby low health friendly target within 15 yards from the enemy target for 100% of the damage dealt.
If the Priest is healed through Atonement, the effect is reduced by 50%."
Why is Penance also included - this makes it so that it is pointless to ever directly cast penance on a friendly target (other than yourself) because you could cast it on an enemy and also heal a friendly for 100% of the damage. This is given that the MoP Penance tooltip currently uses the same value for damage or healing.