Why is Bartz so bad? Wind Shear for capping Elemental chains vs. Luneth cut through and Olive TS?

I'm a new player just starting to get my units into 6* and have Bartz at 5* max lvl. I've read very mixed posts about him, however I was wondering if its worth bringing him to 6?

Additionally, Since wind shear is a 5.4x elemental single target attack, doesn't that make him excellent for capping bosses? Luneths cut-through only does 2.5 + 25% and Olives TS is 2.5 + 50%. Is his attack that low? Is the multiplier not worth more than the at least 25% def reduction?