Lawyers

Lawyers may do good if you hired them.
They are definitely mean if someone else hired them.
Thus they are either definitely mean or may do good.
In other words they are averagely closer to mean:

( "may do good" + "definitely mean" ) / 2 = "mean"

We may need them sometimes but I don't think their job is productive. It is like soldiers, which are powerful but don't produce any.I wish not to face them.