Summoner needs a slight buff.

[Februs]

I don't know why they don't use full Piety+Crits builds, much more useful in pvp than Spell Speed or Determination. Even worse if they have all Vitality accessories.

I don't heal PvP, myself, so I could be way off on this one, but I imagine this had a lot to do with resource management and burst potential. Bursts are not like tank busters in a raid. You're not getting the damage in one single large hit. You're getting it in multiple smaller hits. Having a faster reaction time is more useful in that situation than a flat out stronger heal. Further, having a bigger mana pool to draw from doesn't mean much if you're not casting fast enough to heal through the bursts.

I experience this same thing myself, though to a smaller extent, on Pld. Clemency is a slow and clunky 2 second cast bar. I can use it to supplement a healer, but I can't use it to save someone from a full burst. It's just too slow. It also burns my Mp really quickly, which makes mp regen a pain. After a heavy burst of healing, I really rely on downtime to allow for a natural regen. Any time the enemy is forced to retreat back into their base is invaluable time for rest and replenishment. I can't say for sure, but I imagine that most healers have similar concerns when healing in Feast. Striking a balance between speed and available resources would be something to consider when putting together their builds.