So, Interject...mostly useless?

[Hezzlocks]

One thing to take into consideration, stuns have diminishing returns, Interject, afaik, does not. While it's true that enemies that have interruptable abilities are often prone to being stunned, and in the cases where an interrupt is very important you should be holding back on the stun for that moment, it's still something to be considered, as if the mob becomes stun immune, because, say, the WHM is going all trigger happy with Holy or something, Interject will let you interrupt an ability you'd rather they didn't get off.

Personally, I'm having trouble finding any meaningful use of Arm's Length. There just doesn't seem to be any knockbacks that are affected by Arm's Length, at least none that are worth the effort of tracking. Should be noted I play ranged phys, and am only just getting into tanking, so I've not got any experience with any knockbacks that tanks alone have to deal with, if there are any.

EDIT: Oh, and while the situation has never come up afaik, it's quite possible that Interject will be useful for if you ever want to move a spellcaster mob. Stunning them holds them in place, accomplishing nothing, while interrupting them will usually get them to chase you for a very short while.