But wouldn't Sanctuary be the better option during such types of phases? PoM, CoH and PoH will still be your top heals, and having +25% healing to your DI PoMs seems like it'd be better.
I guess it just depends on the severity of the tank damage/raid damage, how much movement is needed, spacing of the raid and stuff like that. From the perspective of the raid damage component, I'd say Sanctuary would be the stronger option. But in many cases Renew in Serenity can get the job done sufficiently enough to ensure nobody dies. And by staying in Serenity you have significantly more oomph from your Renew/single-target heals, the Renew refresh mechanic, HW:S for handling the single-target damage component, mobility, etc.
Yes, you could stay in Sanctuary and rely on DI PoM and the weaker single-target heals for handling the single-target damage component, while enjoying the better AoE healing. Many times this ends up happening. Where it can potentially fall short is when you have a spread out raid, you have a lot of movement or tank damage is the greater concern (or all 3).