The problem I have with this general approach is that in cases where there is a need to be a flexible healer (10 man raids are a great example), why would I be Holy? Holy excels when there's huge amounts of AOE damage flying around, in which case chances are good that I won't be able to help much with the tanks.
How are you using your serendipity stack? The only boss I've found a use for Prayer of Healing is Lady Deathwhisper P2, covering frostbolt volleys and the occasional melee fail at spirits. Otherwise the damage patterns just didn't support 3 or more people missing 5k+ hp in the same group at the same time. Especially while 2 healing 10 man as Holy with a Holy Paladin, I found myself dropping Serendipity stacks into Greater Heals on the tanks. The incoming damage is less spiky than it was (at least in the current easy mode), and I found I could consistently land hasted Greater Heals with little to no over-healing to help healing would get a little behind (Paladin impaled, Mark of the Fallen Champion, missed frostbolt interrupt, etc.)
On any fight which calls for flexible healing, I simply swap to Disc; Penance and PWS are far superior for reactive healing than Serendipity + GH could ever hope to be.
The best case for a flexible Holy spec is if you're running with multiple Priests in a 10 man group; if you do, then speccing for Divine Fury and Empowered Healing is quite a reasonable decision.