There are a lot of things that lead to variations

There are a lot of things that lead to variations between classes - some shine in some situations, others shine in others. Thok, for example - the interrupt mechanic is less of a bother for some specs than others, but that's just one fight, so it's not a huge deal. There are specs that are better or worse on Blackfuse's belt, specs that are great for dealing with Engineers on Garrosh and specs that are bad at it, and so on and so on.
po world of Warcraft
All of that is well and good, and should be preserved.

However, movement is not a niche. It comes up on most every fight. Being able to deal with momvent is as essential a part of the toolkit as AoE. If some specs can do it and others can't, it's not that you just adjust for one fight - it's something you have to adjust for over the long term, because it's going to come up again and again.

If they put much less movement in fights, that would also work -- but that would also lead to a more boring game.