On the other hand, the relevant question isn't whether a gnoll could make use of an acid arrow, the question is where would he obtain it? Unless we are positing that they order them through Acme-Coyote Mercantile, they have to obtain the acid, manufacture the glass container that contains said acid, as well as some means by which to attach said glass container to an arrow in a manner that doesn't interfere with firing the arrow. Not sure that gnoll society has actually provided for the development of these crafting skills and techniques. While there may be a shaman here or there that has learned how to work with acid and glass - they would be in short supply, as would such arrows. While they might trade with more advanced societies such as goblins or orcs, again they would only have limited amounts of such arrows - not enough to arm every gnoll, dick and harry archer with them. I think the use of such things needs to be toned down in certain situations - a bunch of drow would have all they need, as would a group of bandits, a small pack of gnolls should have maybe one arrow they looted from an earlier victim, but not an armory.
