Okay, for the record, let's go over everything:
1. Nothing anyone has said to date needs to be taken as gospel truth. Their first concern is making a successful movie, and to the general public, no publicity has happened yet, and won't happen until much closer to the premiere date.
2. It's more likely than not that Cumberbatch's character is an existing character from TOS. After all, Abrams made the whole movie series about known characters. He could have invented all new characters but he didn't, and why? Brand name recognition= butts in seats.
3. What villain has the most, heck any, brand name value? Khan. And that's it, no other Trek villain from TOS has pop culture awareness.
4. But can Khan work as a villain in a two hour movie? Tricky. He will be starting from zero since his backstory is "bad guy frim Eugenics Wars" and that's it. The audience needs to be told: What is the Eugenics Wars? What was Khan's role in it? Why do the charatcers care about this part of their history? Why should the audience care? And by the tme you've gotten through all that blahblahblah, the audience is bored and has gone to the lobby for popcorn.
5. The biggest problem with Khan is: what makes him dangerous? he's just some guy from a previous time period. Is he dangerous because he's super sexy? Super persuasive? That's not the kind of threat that translates well to a summer popcorn movie. But an insane guy with superpowers and glowing eyes, now we're talking!
So, Khan has the advantage of helping promote the movie, but trying to shoehorn him into a fast-paced action movie is problematic. He could be rewritten to make him more action-movie-ready, but why bother? That guy already exists.
Plus the edge of the galaxy = darkness is an easy connection. There's nothing dark about Khan, he's a very upbeat guy, until he gets bitter later on but there's no time in a two hour movie to cover all that.