Coming from another Astrologian main who did BOTH Diurnal AND Nocturnal (I flipped based on my partner), my god they were both good in their own right! Regens are handsome, but in Nocturnal stance with Celestial Opposition and Collective Unconscious made for some dang good damage mitigation. Combine that with Aspected Helios, and the party barely took any damage. A single Helios would've topped everyone off after the raidwide.
On that flip side, after the raidwide, combine Aspected Helios and Celestial Opposition and the party would be full or nearly full before the next one. If not, you could throw out your Collective Unconscious for a little damage reduction and a little extra regen.
My BIGGEST argument is that, especially when solo-queuing 8-24 man content, if I had a WHM or SCH was in the party, I could easily swap and complement my healer. I.E. Regens and Shields. WHM/NoctAST or SCH/DiurAST. Making AST a pure (regen) healer is going to make why I chose AST null and void. The cards were just a bonus. I LOVE(D) being able to swap on a moment's notice to complement my partner healer.
I will admit, I did prefer Diurnal stance in 4-man content simply because I could get a new and squishy tank to full quicker, but I loved being able to just swap depending on my healing partner. Basically, them saying they're making AST a pure healer is making me extremely worried.