Because people who have an actual clue about the game would use a non-viable spec while running for world first (and succeeding). Makes so much sense. And that was for Garrosh, for most fights they ran with Val and Viklund. Even for fights where shadow is clearly suboptimal. Why not 2 mages if shadow is "non-viable"? Markmanship hunter, THAT is not viable. Shadow is just not as overtuned as warlocks are, and you just don't know the difference between viable and overpowered. Fortunately the people who have an actual clue about the game do.