In all honesty, Eye for an Eye is really not necessary for end game WHM. Yes, it's good to have. But is it as necessary as Swiftcast and Virus? No. I rarely ever use it in FCoB, and even when I do, it's just a bit of extra fluff and is not a major contribution. Sometimes it doesn't even affect the boss/adds/whatever. Take your time getting it. To be fair its greatest use is for mass pulls in 4-man content. And even then, holy spam will generally negate its chances of proccing, soooo... womp womp.
WHM only -needs- one skill.
Holy.
Everything else is just icing on the cake.