Whilst I'd agree that there are a few occasions where wall to wall pulling either isn't viable or more efficient than single pulls, saying that it's slower in most situations is pretty far off the mark in my book. The only times I'd agree with you are the following:
Very low level dungeons, particularly with certain jobs getting their AoE later than others.
Several incompetent players in the party, an especially bad Tank or Healer can cause problems by themselves. One awful DPS doesn't really make much of a difference to me. Two can cause cool down issues on certain nastier pulls though.
Certain older dungeons that either have particularly lax gating or harsh iLvl caps that will frequently allow you to pull way more than is feasible to heal. Aurum Vale is probably the best example of this.
It's very doable, but it's also very messy and inefficient.