Mercurial set is pertty much useless at the moment, despite the fact that it is the only non-SL-armor unavailable in the Hall of Heroes. Its Movement Speed Increase: Low per piece is non-existent. Acording to the Wiki, MSI Low increases movement speed by 4%, that's 8% for the whole set. The only other armor providing the same bonus, Mercurial Demo set, provides also a BDB med, making it superior in every way.
I suggest the MSI on Mercurial set would be increased to med per piece, as should the 4 star version, Charged Quicksilver set get the MSI low per piece. In addition, there should be either a Quicksilver Shield that could be crafted into Charged Quicksilver Shiled that would yield a MSI low which in return could be crafted into a Mercurial Shield with MSI med; or MSI trinkets (cough http://forums.spiralknights.com/en/node/60148 cough).
These additions would make The Mercurial set a decent choice of armor, alongside the weapon-specific armours.
Mercurial set is part of a token line, Token lines are not acquirable from HOH. *for a silly reason you CAN acquire the 4* Recipes from a mission vendor*
but yes, I completely agree that the Mercurial needs more love. Maybe even just a pox ASI low per armour piece could be useful/nice