Ultimately up until Cloudrest, most trials had both healers run SPC to get the buff on everyone. These days they have access to Olorime. My understanding is that the really good veteran trial guilds just stack and use insane damage to blow up bosses, which means they can run Olorime, Infallible, Sanctuary and Worm's Raiment across the two healers.
Really ESO balance is busted because of the massive variance in damage among players.
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Honestly, for normal dungeons and trials, anything can work. Some of the normal DLC dungeons may be a pain without optimized builts but as long as you have adequate sustain you are OK.
Also if you are healing dungeon PuGs, the significant majority of ESO damage is avoidable. In a good group, the healer spends the majority of his time debuffing and doing damage (and thus ends the fight even faster) and only has to heal to deal with mechanics (which is why Templar is so good, Breath of Life is the best burst heal in the game). But in a bad PuG, you really want the sustain
ESO seems the opposite of WoW in this in that group play is extremely synergistic and building a group thoughtfully (down to using the right spell morphs and micromanaging who uses what set and what debuff) can have significant effect on performance.