They only work on crits. Since as a tank you should be striving to be un-crittable, the talent loses it's advantage on melee hits. As for spell hits, spell criticals aren't as big of an issue (and don't appear that often), as the really big spells can (usually) be interrupted or avoided or something, and the smaller ones can be managed.
Eye for an Eye is strictly a PvP talent, and it always will be.