Clemency does give you security in certain dungeons with big pulls, because if the healer struggles then clemency means you don't need to rely on them, but it is overall rare that a healer cannot handle them if you mitigate and avoid AoE well. I realised from playing the other tanks sometimes that it just didn't matter that I didn't have clemency or Hallowed Ground.
Some examples of dungeons that either have big pulls that still or previously benefit from clemency with certain healers:
Ala Mhigo (healers frequently still in leveling gear)
Kugane Castle (last pull)
The Drowned City of Skalla (pull before second boss, easier now it's not current)
The Burn
Most dungeons don't hurt enough but these are a few examples I've found it useful in multiple times.