After reading a number of posts, and scanning others, I guess I should just stick to my own view on this one.

Sometimes you meet someone is really is a good person, but s/he just isn't the right one for you. Occasionally, that person actually is a better friend than a lover. Other times, you are just at two different points in your life which aren't as compatible as you would like. So rather than going down a path which would destroy any chance of even a friendship, not to mention an intimate relationship, you break it off, and decide to be friends instead. That way the door is left open for something happening down the road if opportunity allows. In the end, it really depends on both people. You certainly don't want to interfere in each others' love life while one or both of you are in a separate relationship. That takes maturity and self-control. If either one of you lacks those things, then remaining friends really isn't going to work. You're better off not considering it.