What is the requirement for this ability to trigger? It says "bonus attack when enemy uses magic" but I haven't ever seen it used.
I did some testing using pvp in the test realm. I had a couple of companions with witch hunter fighting against a witchdoctor with Scratch and Mormo. I positioned my companions right beside the enemy and had them attack me. The normal attack from Scratch, Mormo and the witchdoctor didn't work. The single target spells that Scratch and Mormo have didn't work. A multi spell from the witchdoctor didn't work. Sure, things might be different in pvp (i.e. the talent may not be working) but if it isn't then what is the point of the talent?
I think I remember seeing this trigger once or twice way back when I first started playing. I think it was in my battle with Old Scratch, and it was the NPCs that used it. However, I haven't seen it trigger since. I've never seen it trigger for companions, a couple of which have Witch Hunter.
Witch Hunter is the "First Strike" of magical attacks. If a magical opponent targets a character with the "withchhunter" ability then the character will strike first. I have seen it used a few times and a text appears that says "reduce damage". If this is not a graphical glitch then witchhunter is a first strike that also weakens your opponents attack. It is rearely seen in PvE since relatively few mobs are magic users, but it is a useful counter- Scratch talent in PvP.