Excerpt from "'Should I Buy It?' – Universal Remote & Other Exotics"
Mask of the Third Man is one of five Hunter Helmets, but the only one specific to Bladedancers. Further, it's only one of two Exotics specifically for Bladedancers; the other is Lucky Raspberry. MotTM makes your Arc Blade abilities cost less energy – Showstopper, Razor's Edge, or Vanish. It's a nifty perk and could directly lead to extra kills.
If you're a Bladedancer, it's basically a must-own; Bladedancers only have two Exotics specifically for them. While we also like Lucky Raspberry's grenade buffs, an improved Special is welcome in both PVP and PVE. There's additional utility from Infusion in Crota's End (Hard Mode) where health doesn't regenerate. Grab Orbs and stay alive! The boost to Intellect is a highly-welcome plus.