I'm always open to being wrong, but stating I don't understand how it works given your use case....
The whole point of mobility spells is to mitigate lost dmg from having to move. If you are sitting on Shadowflame stacks while you're not moving so that you can get 3s of mobility when its needed you are losing dmg before movement is even brought into the picture. That's up to ~8 imps per minute you didn't spawn because you spent those shards on Stalkers. If you are stacking the DoT correctly so that you are not at a net negative in dmg from picking the "mobility" talent.. then you have no charges when you need to move. I don't understand how you can view its as a mobility talent.
We've always had lifetap/doom for stutter stepping movement and can Demonwrath for extended movement. Now that DC can be procced by Demonwrath IMO its really not even comparable anymore if you're looking at it from a movement perspective.