Take a look at my Rogue:
Best in slot show me to use:
The Empty Crown - one of the most weak Legendary
and neck enchant: Mark of trained Solider
What is wrong?
Well, I don’t think there is anything wrong. If I do a BiS with all gear available and simulate it, here is the BiS that is suggested by default:
If I lock in Mantle of the Master Assassin for shoulder, I get this:
So, it looks like the optimizer is correctly choosing the gear that simulates to the highest DPS. Maybe that legendary isn’t as bad as you thought?