Look, I think it's really about what you are able to deal with.
It is great to remain friends with your ex if you can. But for some people it's too sad and it does inhibit healing and moving on. It also depends on the circumstances.
When I broke up with my ex, we continued to see each other. It was a mutual decision to separate and I felt enormously sad, although I knew, without doubt that it was over.
We were, after a while able to talk through the things that happened, and I think that we both found this comforting.
We remain close friends to this day and I don't regret for a moment having been in a relationship with him. It was the friendship that saw us through.