while arpen at the moment is a "suboptimal" stat, with expertise & hit being capped faster very patch, and haste reaching some kind of wall ( at least from what i´ve heard ) at ~600 haste, you´ll be kinda like forced to have some arp at some point.
maybe at a certain point of arp-rating, it gets good enough in stat value to be a viable stat to stack, similar to classes like marksman-hunters, warriors, combat-rogues, or ferals.
i dont know if that point actually exists, at which point you will reach it, and at which point will will have "paid off", starting to show profit.
guess we will see after we got a full bunch of arp-gear