At the very least, what needs to be fixed about is its duration.

Recasting a buff like this on Paladin/war is just tedious. Generally speaking, if you're gonna use Defender, you're going to keep it up full time, not up sometimes and down other times.

Take Scholar's Dark Arts / Light Arts. Scholars do not need to fool around with using the same Job Ability over and over again; Light Arts stays up almost for as long as they need it to.

Popping Defender every five minutes is just meaningless, repetitive button pushing adds nothing to gameplay. The duration should be increased to until the player zones or dies, in addition to any other adjustments.