Old reply, but, worth considering, if fight grants exp, or any rewards at all, it's separated from the usual optional content. If this hypothetical pay-raid gave even any exp at all, it's experience available only for pay, giving paying players an advantage. And since the fight would at minimum probably increase spiritbond, there's almost no way to remove rewards from such an idea. Somehow come up with an idea to remove even that? You'd have to remove Player Commendations, too, or they'd have an extra means of completing a Challenge Log entry.
The whole point of the "optional" is that it grants no advantage over non-paying players, I should think.
