Having Mythic only level dungeons that don't have to be tuned around 10-15 minute easy mode LFD runs is great. It means they can actually make dungeons feel like proper experiences again. You could never have a 9 boss epic 5-man like BRD or the new 7.1 concept of Karazhan if it had to be tuned for LFG and included in the random dungeon rotation. It'd have to be split up into shitty 3 boss wings.
I'm all for Mythic only dungeons.