Its fine how it is, the most WHM should just learn when they should and when they shouldn't squirt the mob away.
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If you wait a moment before casting your next action the knockback canceling your gcd is less likely to activate.
In dungeons, to not piss people off, you use it as a last hit on mobs that it will straight kill or will soon die to dots.
It's completely wonderful the way it is, and can be used strategically on mobs the knockback affects. Otherwise it should always be on CD on bosses.
Honestly my biggest gripe is that the devs are making more and more monsters completely immune to knockback. It's funny when in early 2.0 areas everything seems to get knockbacked.