I feel it is very bad. Because there is obviously a bad reason as to why you broke up ( heart break, cheating, fighting too much etc.) and if you stay friends with them, you get closer, and closer, and closer friends. Then eventually you become so close you start to develop feelings again. Then sooner or later, you guys break up, someone's feelings get hurt, you two hate each other. And it just all ends badly and it makes things awkward and different between you too. I wouldn't take the risk.
It's not if you can do it. I have one that constantly tries to play mind games with me and it drives me nuts. Had to block my e-mail, Skype, and my telephone and I've heard she's followed me here also.
My other ex and I parted ways with a little animosity but it will be fine. I'd like to be friends with them but one just doesn't want to. So be it.
It can be bad if they're controlling or if either of you get jealous. But personally I wouldn't be dating anyone unless we had already been friends before. So if you happen to not care for dating that person and you want to remain friends, I don't see why not. There needs to be some type of mutual agreement in these situations.
It's not bad! You just stay friends with them because if you leave them totally then I'm sure that may turn you guys into enemies so to prevent this a decision is made to just stay as friends.
It's bad because then they'll want you back even when you're with someone they will try to do all in their power to split you up to go back out with you