Blizzard has prevented the use of cross-realm grouping for current-tier normals/heroics. The reason I always hear is because it would destroy guilds (no idea if that’s true.)

“Because it would destroy guilds”? That sounds like the dumbest reasoning I have heard to date. I imagine their reasoning may have more to do with the competition involved in server first kills would be destroyed if someone pulled a group of competent raiders from another server and had them do the job for them but claim the reward. 

So I guess in a way, yeah, that “because it would destroy guilds” thing is a legit concern.

Ahh here it is from the crab, with much more détante: