- Maths behind zero FrR for Druids and Priests
Sapphiron's Frost Aura deals 300 DPS before any mitigation from Frost Resistance, abilities, or talents. Each point of FrR reduces DTPS by 1/420 = 0.24%, and hence reduces damage taken by 300/420 = 0.71 DTPS. Let's conservatively assume that incidental damage from other frost damage sources (Icebolt) increases that to 0.8 DTPS.

Compare this with how much HPS a point of +healing gives you. All stats assume 30% spell crit.
* Rejuvenation (base): 0.8/1.5 = 0.53 HPS
* Rejuvenation (8p T2): 1.0/1.5 = 0.67 HPS
* Healing Touch (rank 4, Idol): 0.45 HPS

* Renew (base): 1/1.5 = 0.67 HPS
* Renew (5p T2.5): 1.2/1.5 = 0.8 HPS
* Prayer of Healing: 0.55 HPS
* Greater Heal (base): 0.39 HPS
* Greater Heal (8p T2): 0.79 HPS

Looking at expected strong strategies for raid healing on Sapphiron, for Priests, we can expect 8p T2 and 5p T2.5 to be strong due to the sustained raidwide damage. Priests will spam either Renew or GH and mix in Prayer sometimes. For Druids, 8p T2 or 2p T3 will be strong. All these strats give around 0.5 - 0.8 HPS per +healing. Let's conservatively assume that 1 +healing will only give you 0.5 HPS - perhaps due to overhealing (should be pretty low), you're a priest with Renew banned (that's stupid on this fight), or you're a 2p T3 druid to help your raid with mana and energy regen.

Thus, conservatively, 1 FrR reduces DTPS by 0.8 while 1 +healing increases HPS by 0.5. Now let's look at the gear choices available. I'm comparing less-than-BiS +healing pieces to BiS FrR pieces. For simplicity, I am ignoring stats and mp5, but obviously, those will also be better on healing gear.

Rings:
Icy Depths (pre-p6): 20 FrR = -16 DTPS
Ramaladni (p6): 25 FrR = -20 DTPS
Caut: 46 +healing = +23 HPS

Trinkets:
Ice Reflector (pre-p6): 15 FrR = -12 DTPS
Seal (Priest): 33 +healing = 16.5 HPS
Seal (Druid): 44 +healing = 22 HPS

Necks:
Tempestria's (pre-p6): 15 FrR = -12 DTPS
Touch of Frost (p6): 24 FrR = -20 DTPS
Evil Eye: 44 +healing = 22 HPS

Cloaks:
Master's Cloak of Frost Resistance (pre-p6): 15 FrR = -15 DTPS
Glacial Cloak (p6): 24 FrR = 20 HPS
Hide of the Wild: 42 +healing = 21 HPS

As is easy to see, in every slot, the +healing gear wins out, even comparing phase 1-4 healing gear to phase 6 BiS FrR gear. If you look at p6 crafted cloth armor, that actually starts to be competitive with pre-p6 healing pieces - but obviously, those should be prio'd to caster DPS, and when time gets around to you possibly getting FrR, they'll be competing with BiS gear from phase 6.