A foe hasn't officially "used" a skill if the skill didn't complete activating.
When Wastrel's Worry is applied, a 3 second timer is activated. The damage is applied after that timer has expired, no matter how many times it is reapplied. At this point the hex ends, so any reapplying during the 3 seconds is a waste of 5 energy.
If ended or removed prematurely, no damage is dealt.
If the target foe uses a hex removal skill on themselves, Wastrel's Worry ends prematurely. The next hex after Wastrel's Worry is removed instead (if there is one).
A good "pressure" skill in many situations. When opponents see this hex on them, they will often panic to use a skill before it ends, opening a window of opportunity for interruption, Dazing, Spiteful Spirit, and such.
This skill is peticularily effective against foes that do not use skills or uses very few skills. Ranger pets, Necromancer minions, Kurzick Juggernauts and Luxon Siege Turtles are good examples of such foes, along with many Dungeon bosses.