I don't find it strange to still be in contact with your ex is the relationship ended on a good note. I think people think badly of it because they're too insecure about rekindling and what could happen. If the relationship ended, that means that the two people weren't compatible as partners. It doesn't really mean they can't be good friends, they're just no good as a pair.
I still talk to one of my ex's from time to time. After we broke up we still spoke quite often, in fact just as much as when we were together. We just happened to grow apart over the years and that's the only reason we're not in constant contact anymore. I did keep in contact with one of my other ex's, but the same sort of thing happened and I lost his number so we don't talk anymore.
If it ended badly or one side was really clingy and didn't want to break up, I don't think it's good to stay friends. It will cause unneeded heartache and might never feel right to be around that person. If the couple broke up for a more serious reason like cheating or abuse, they should stay apart period.