unusable Epharim?

Ephraim's strongest trait is his ability to buff multiple stats with a combination of his weapon, C slot, assist skill, and seal. He honestly can have any IV and still be useful as just a buffer.

Absolutely. Ephraim is one of the best buffers in the game, especially for Bladetome users who require hone-type buffs. Put him next to Nino and she becomes superpowered - especially useful for multi-level tasks like the chain challenges or squad assault, even if you don't end up using him in combat.

That being said, at least his attack is still fantastic, so even in combat he should be pretty good.

His main job is to buff, so you're OK. With TA and QR he'll check reds at ease.
I've been using a +HP/-Spd Ephraim for months in arena and have no complaints about him.

Mine is +spd/-res, even without +atk he hits 51 dmg unbuffed. If you were planning on using him for combat, he's still very viable.
On the other hand, his primary use is for buffing anyway, as others here have mentioned.

Whilst you're at it, pair him up + support him with any real bladetome user, and go to town with em.