The point of Me x4 is that in a mirror match, the AI just looks for whoever's gonna take the most damage from a disc (with consideration for expected deaths and not counting Damage Cut bonuses), and MeX4 provides SO much defense boost that with it at lv50, whoever you give it to will probably be targetted LAST provided you don't position them in blasting range of someone else.
So you don't use it because defense so much as to game the AI to not attack a certain girl first. (So, it pairs pretty well with Perfectly Imbalanced if you're short on good offensive memoria) Since it also provides Damage Cut which is usually better than defense, it's probably the best defensive option apart from the taunt+def memo Sunlight Shining Down. (It'll also give notably more attack than 3* defense stat options)
I'd rather just equipment the strongest offensive memorias I have, but it probably has use in some situations. And yes, Kyuubey Knight is one of the best passive memos. A charge-boosted blast disk does a looot of damage in mirrors with Charge Up bonus, and it's got +25% attack and good stat split. If you're using a puella combo, charges do as much damage as accele, so it's pure profit. A maxed My Bible is also pretty good so you wouldn't notice a huge difference, but it's probably a little better due to Charge Up, plus do you already have more than enough lv50 offensive memos?!
