I had a similar experience in Lost City of Amdapor once. The tank speficially asked me to not stoneskin him as it was "useless" and a "waste of time". I agree that they could have made it slightly more potent, but on WHM it's a 18%/max hp ratio, which, depending on on the tank is up to 1.5k/2k hp I don't have to heal. So I wouldn't exactly call it useless. I tend to just put it on the tank the last few seconds of each battle during speedruns, or on the entire party if I have time if I know a big DMG AoE is coming up. It definitely makes things easier for me as a healer.