Since only one bothered to actually give a reason as to why single pulls are justifiable, I'll ask a different question. Why is it the tank the one who sets the pace of the group? Why do some say it's the tank's choice to decide the pull size?
Just because the tank is the one at the front taking the hit from mobs, doesn't mean they're automatically the leader of the pack and decide that. The leader can be the one at the front of the line or at the very back coordinating. And with a group of randoms, you can't just assume you decide how they play because you're a tank if it barely has an effect on your game play.
Logically, you'd let the person who has to put more work with each pack of mobs decide that (AKA the healer). If the healer can keep you alive
without you having to do extra work when pulling an extra pack, then what's the problem? Let's even assume an extreme case. If a healer can heal you and keep you alive in a wall to wall push
without you having to do any extra work, then why would you refuse? Wouldn't you pull wall to wall in an unsynced dungeon because you know the mobs can't kill you? (we're excluding the damage increase here, since damage isn't the issue people are discussing).
So saying the tank should be the one to set the pace is illogical, since the healer is the one doing the work.

(Image source:
https://www.reddit.com/r/ffxiv/comme...ho_pull_small/)