I don't know if this is really underused because I don't PvP and I usually just group with henchmen for PvE. But I have been ignoring this skill for a while...
My mesmer has recently started using Arcane Thievery and it's been quite useful. Yesterday I stole Animate Bone Minions from a Necro boss, which basically shut him down. It's fun to use spells from other professions, but even if I don't use the spell I steal, keeping the baddie from using it helps a lot.
Underused skills
wwwgeek7