As a PLD main, I hate it when people use knockback skills or don't put their Garuda on Obey. It really messes with my ability to keep aggro off the healers, even in low level content. In high level content, AoE is king and knockback messes with that too. Also, personally, I'm one of those tanks who rotates targets to build enmity on all of them at once, so I'll split my combos between them as I go. Not only does that let me keep aggro well off the healers, it lets me put my damage mitigation move (strength down for 20 seconds) on all enemies. Also, by building up extra enmity on the non-main target when there is a main target, when the main target dies, I have enough aggro on the rest that I can switch to my MP-restoring move (and DPS moves if 52+) to get ready for the next pull.

In my personal opinion, the only times to use a knockback move are:
  • The tank lost aggro and the enemy moved far enough away and in the right direction that, if you knockback, it'll push the enemy right back next to the tank where she can re-establish aggro without having to move. If the tank has already started moving toward you, do not do this! You'll overshoot and its annoying.
  • An enemy is ranged and won't move, and you are lined up perfectly to push the ranged enemy to the tank (it was extremely nice in the old DD first room farms with those two ranged enemies that were hard to move even with LoS tricks). Or, similarly, when an enemy needs to be somewhere specific and it isn't there and you are lined up just right to push them there.
  • An enemy is already against a wall and positioned such that a knockback move will not move it.
  • An enemy needs to be interrupted and you're rather sure the tank won't be able to do it in time and no one else has a stun. There are also some stun-immune enemies that are not knockback-immune and can be interrupted via knockback, but I'm not exactly sure which.
  • You are fighting a knockback-immune enemy, such as the majority of bosses.
  • PvP.
Any other time, knockback can be incredibly annoying to tanks and melee and can result in the healer dying, especially if the tank fails to notice that her AoE threat generators are no longer hitting one of the enemies.