You don't ever need to, it allows either wheel to turn faster any time. And once an ARB is bound up from a turn, good luck unlocking it. Better plan ahead, no need to with the detroit.
My point is simply this, an ARB can either be an open diff or a spool, it can never be a Detroit(which is more like a ratchet). A detroit is the best of both at the same time in my opinion without all the extra air lines and complexity. Lets say you are climbing up a hill while turning at the same time. A detroit can send the power to the inside wheel with the most traction while at the same time allowing the outer wheel to freewheel faster around the turn. An ARB cannot do that. You get to select whether you want to spin the outside tire and not go anywhere, or bind it up and possibly need to jockey it around the turn with some forward/reverse action.
If he likes the G80, then stick with it. You can't put a detroit in a G80 carrier anyway in a 14BFF. They are different carriers. Or maybe he doesn't have a 14BFF, I don't even know.