Most Helpful Guy
I think one of the main negative result of making your ex a friend again is your new bf/gf.
If you become good friends again, you would obviously have to tell your new bf that he is your ex.
This would probably not sit to well with most guys as they would always be constantly worried that you may potentially leave and want to get back together with him again. That they are just a stepping stone to take back the ex.
It is just an undue strain that your new relationship does not need.
If you don't have a bf now... that may become and issue someday when you do get one... by then you and your ex may be very chummy, and if your new beau does not like the situation, then you may have to consider dumping your ex... yet again!
It is no big deal if you are just being friendly with your ex if you happen to bump into them... but putting them in the friend circle again does more harm than good.