Most Helpful Guy
Because when I dated someone it was for more than an activity partner, easy access to sex, and someone to bitch about my day to. I dated people I also admired and respected for qualities, values, and goals that had nothing to do with being a good girlfriend. Because of that, just because they proved themselves lousy at the latter doesn't mean they can't contribute to my life in other ways, or I to theirs.
So, it would be hypocritical of me to say a partner can't be friends with her ex. What would tick me off though is the crap that people try to pull where they're going to a club or party together without her boyfriend or she wants to stay with her ex when he comes to visit. Out of respect I would never do that to someone I was dating, and I would drop anyone who disrespected me in that way.
It's interesting to me that almost everyone expects "The past is in the past" to apply to all of their hookups, even when they might still be in position to hook up with them every day, but remaining friends with an ex makes someone completely toxic.