No one is going to deny that evil people exist, so what's wrong with giving them their well-earned label? You're just worried Ogron that they're gonna spread the evil descriptor around like peanut butter and innocent people are going to be wrongfully convicted of crimes.
It's not that I worry about this standardized idea of evil getting innocent people convicted on its own. You say that no one is going to deny that evil people exist but that's not the problem here. The problem is figuring out exactly what that evil is. (And depending on the nature and subject of the discussion I can easily imagine myself trying to deny the existence of evil.) As mentioned in the article - though it's an extreme example - terrorists bent on fighting the US and western world have a very different idea of what evil is. My point is that even though we are more alike than an american and a terrorist, evil probably means something very different to you than to me. This means that when a Doctor Something tells me so and so is evil I will think of what it means to me and not what it means to him. This again means that in the case of having a judge/jury understanding what's wrong with a criminal you will lose something and not gain something by putting the evil label on him.

In my oppinion evil is mostly a religious and emotional term. It helps us feel better when a person is killed if we 'know' he was evil because we know we were justified in removing him from the world. The justice system is not supposed to deal with these things though. It's fine for someone working with these people (police, psychiatrists etc.) to tell him/herself that the man was evil - but IMO it has no place in the courtroom.