I don't think that they'll hurt multiboxing very badly. I do get the feeling that they don't have a problem with it but don't want it to become too prevalent, but the game as it currently runs seems to do that just fine.

My concern is the same as supernova's; as long as they keep the game within reach of casual players, I'll be happy. I'm on a server with friends again and am in a guild they joined, so I spend more time single-boxing these days. As long as the game remains fun and there is a reasonable progression path, I don't think this expansion will make me leave.