In a number of cases, cheating is a sign of emotional immaturity of not being able to confront issues head on. It's like the employee at a company who hates his job but doesn't have the balls to tell his boss and see if he can make it better -- it's just easier and more convenient to not confront it and take another job. Often in those cases of both relationships and employment, you're typically dealing with someone who is unable to escape themselves -- as the same or similar problems appear with the next girlfriend, the next job, etc.
In cases like that, the cheating isn't so much an emotional attachment as it is an escape, as Kate mentions. If the cheater can realize how f*ed up they are in being able to manage themselves and confront problems better directly, that could be reason to take them back if the offended partner otherwise sees too much good to lose along with the bad. But that partner sure as well won't trust the cheater, and the cheater is going to have to earn it like hell ... and may never get full trust ever again.