I suspect it'll see as much use as it currently already does in Naxx: Not much. It's not powerful enough as an ability to outdo either Hammer of the Righteous or Shield of Righteousness, so those two are out already. Holy Shield is also something you don't want to replace normally, though it'll be useful as a replacement on a boss which only spams non-interruptable spells. Judgements have additional benefits to using and are nearly as strong in damage done so are unlikely to be dropped for it either.
That leaves Consecration, Consecration is more powerful on average if the mob in question will stay within it's effect for the full 8 seconds, but on a boss which this isn't guaranteed to be the case it could be a useful replacement. The cooldown on Exorcism means you'd only be able to replace it once every other Consecration however. Grobbulus is a good example of a current boss in which you would be better off alternating Exorcism and Consecration.
It will definitely be useful on something freshly pulled though, since you can usually get in both an Avenger's Shield and Exorcism in that time, which is a nice headstart on threat.
Exorcism does have an additional downside compared to some of our other abilities in that it uses the spell hit rate and not the melee hit rate. It'll thus still have a small chance to miss even if you are melee hit capped.
Finally, there's the obvious utility aspect with
[Glyph of Exorcism]. It'll still be an interrupt on the GCD, but it's an additional interrupt we can use on anything which can be very handy at times. That is of course assuming that Glyph remains unchanged for the new Exorcism.