Originally Posted by Malan

Not many weapons will be affected by this.
Weapons - Items - World of Warcraft

We can be sure that the change is only going to affect melee weapons, since the recent trend has been to specifically change one-hand caster weapons to main hand only, to prevent shamans from abusing DW with the Elemental/Resto tree.

My thought is that all this really does is allow BS weapons to be used in the offhand.