Spirit its the worst stat because some FL fights require a lot of throughput, it happens many times that your raid wipes and you still have plenty mana but you didnt have enough throughput to keep them alive.
Harmony its a really interesting mechanic, with the crit buff Regrowth places itself as a really good option to refresh our mastery.
Why? well, couple things
With the 200% crit thing, it heals for more than b4
On the FL raids, druids are perfect raid healers, meaning that you want to cast single target spells just to refresh mastery and then keep rolling HoTs, regrowth its the fastest cast, meaning that you can max your throughput by using it.
It consumes clearcasting
What i do its keep the mastery up by using Swiftmend and Regrowth (hopefully with clearcasting), because most of the time you dont have time to cast a HT or Nourish without putting some risk in your raid.
Also, i find myself refreshing my LBs with just another LB cast, since rather than throwing a long cast on a tank who doesnt need it i prefer to use only one GCD and keep working on the raid, and use the SM to heal someone that really needs (including effloresence)