Well, there's two ways they can go about the horde mode. Have it purely for fun, offering no real rewards. Grinders will write it off as pointless. Or have it with rewards of value. The grinders will like it, yet those just chasing some fun will find it will probably be restricted like the Nightfall/raids are. Of course, Bungie could have it with a decent reward system, that would not break balance, nor seem useless, and thus not need to be restricted. But going off their "reward system" thus far, that's not going to happen.