One of the things I’m most excited about in Legion is the focus on class identity. The fact that every spec gets some lore surrounding their artifact weapon is pretty cool to me. I don’t expect them to be ground-breaking quest lines (since they have to do 36 of them), but the very fact that each spec gets a little something unique means a lot. It puts a lot more weight behind your choice of spec, and sort of commits you to that choice. I think fewer people will be inclined to switch to “flavor of the month” specs if they’ve identified themselves as another spec already.
However, it does put a little more responsibility on Blizzard’s end to make sure classes with more than 1 DPS spec are relatively balanced. I think it’s important that every time a new patch or hotfix drops you don’t feel forced to abandon your artifact journey because of numbers.