The extra healer dps afforded by pld's extra mitigation is still less than the dps difference between skilled pld and skilled drk.

One thing that hugely improves tanks and healers' dps that the majority of players ignore is doing multiple tank swaps so both tanks' defensive cds are fully utilized, allowing them to stack multiple cds for every tank buster, which makes healing them much easier despite the little/nonexistent tank stance uptime. Everyone seems to be so stuck in the mindset that pld/drk are the main tanks and they should tank the boss as much as possible, and the war only takes the boss when there's a mechanic that requires that (chastening heat, gobslice). The same thing seems to be happening between ast/whm and sch, where people want the sch to dps as much as possible while the ast/whm does so much of the healing, instead of sharing the burden and coordinating buffs/cds to maximize both healers' dps.