He's not useful in much, the problem is with only 5 health at the 6 mana stage a bunch of stuff can trade with him. The reason why Violet Teacher's good is because at the 4 mana stage, the only viable cards that can kill Violet teacher in 1 shot are Ancient Brewmaster, King Mukla, and Leeroy. If you wanted a 6 mana card, the slot is much better for Argent Commander, Sylvanas, Cairne, Sunwalker, Black Knight, Highmane if you're Hunter or Fire Elemental if you're Shaman. Any more cards at the 6 mana cost would make your deck heavy at the top.
tl;dr disenchant but only if you're using the 400 dust for something immediately otherwise save it for when you need it.