Hey there, Neo here.
To the point with this one: Does the Mercurial Set have any real use? Sure, the shock is nice, but the road sort of ends there, unless movement speed is really, really helpful. Considering how rare movement speed boosts are, I would not know!
Additionally, with pet perks, I believe an additional Medium Movement speed is possible -- meaning you could have Very High movement speed with the set and a level 100 pet (Again, worlds away.) The Backfire Barrier increases your speed even further, although the Scorching Barrier is much nicer for damage, I hear.
Thoughts?
-Neo
Very High movement speed is just as useful as Very High attack speed and Very High damage increase is.