Having a look at the logs of a primarily single target fight like Sisters of the Moon, there is clearly a large difference in single target output between the specs:
https://www.warcraftlogs.com/statistics/13#boss=2050
All else being equal, do you think that it's reasonable to decline the bottom two-thirds of the specs in that list in order to maximise the probability of successfully clearing the Tyrannical dungeon in time, given how crucial single target DPS is this week? A group with, for example, an Elemental Shaman + Assassination Rogue + BM Hunter vs an Enhancement Shaman + Subtlety Rogue + MM Hunter could be the difference between a key downgrade and a key upgrade. And it makes sense that a prudent group leader would do whatever they could to ensure success.