I thought about the hacking issue some more, and you can pretty much foolproof this even if you're dealing with an ability with a wide range of damage. Let's say WotL does 1 to 9999 damage (random), what the server can do is simply compute the result of WotL as it's casting the spell, so as your client gets WotL rendered on your screen, the server also tells you 'Psst, in case you suck at dodging I've already determined WotL will do 9999 damage to you'. This means the client now knows for sure exactly what damage to display in the event of failure and it can be sure this number is correct, because that's what the server told it to be. You can't change this number with hacks because the server already knows what the damage ought to be! Not counting damage reducing effects, any number other than 9999 will immediately tip off the server that you're hacking your response.

So my conclusion is that using this model to stop hacks is needlessly paranoid. And if there are supposed to be other advantages for using FF14's model over the conventional one, I sure can't find any.