I prefer to remain friends with former partners, and I'm willing and able to do that. I've done it. In fact, my former spouse is one of my closest friends. Although we now live in different states, we still talk most days and visit at least a couple of times per year. In fact, he spent two weeks at my place over the holidays. I'm also friendly (although currently not close) with a couple of other former longtime lovers.
My former spouse and I agree that if we hadn't decided to be poly several years into our marriage, learning all the skills and doing all the self-examination and exploration that entailed, we probably wouldn't have remained friends, let along close friends -- and that would have been a huge loss.
As far as I've seen, what most people expect to happen in the monogamous paradigm is that if you've found a partner, that's such a privileged and venerated state that you're obligated to stay in that relationship unless there's a "good reason" to leave. Which is why people often stay in mono relationships until resentments or boredom or despair accumulate to the point that there's some kind of catastrophic breakdown, a lot of drama, and somebody leaves in a huff feeling righteous about it. It's like you have to achieve sufficient anger or desperation to achieve escape velocity -- and to be able to justify that decision to others.
IME, the amicable "we just grew apart" is rare in mono relationships -- at least ones that have progressed to the level of living together or life partnership. It happens, sure -- but not as much as the blowups and subsequent blow-offs.
More importantly in a mono-dominant society, most people *expect* that exes can't or won't really be friendly. That's why people usually express surprise: "Oh, isn't it great that you two are still on good terms!" That common, brief reaction speaks volumes about social norms for intimate relationships that have ended (or, more accurately, transitioned). I get that reaction frequently -- and while I know people intend it to be positive, it bugs me. I wish our society wasn't like that.
I agree with the earlier poster that whether former partners remain friends is mostly a matter of the people involved -- and especially, IME, how willing they are to own up to each other. You can't transition a relationship that can't heal, and a relationship can't heal when big gaping wounds are left unaddressed. Some people are really willing and able to do this. Some aren't. Including many poly folk.
I blogged about owning up after a breakup here: http://solopoly.net/2012/12/24/the-g...up-guest-post/
I had two breakups in the past year: One short but intense connection with an ostensibly mono guy who claimed to be open to dating a poly woman, the other a 3+ year relationship with a married longtime poly guy. Both guys, quite frankly, summarily threw me under the bus rather than work with me to resolve some issues. Both, intriguingly, also claimed at the time that it was important to them that we remain friends, that they valued their connection with me even though the intimate relationship had to end.
In both cases, I said fine -- could we clear the air here about what happened and what we each contributed to the situation, so we can move on? And I laid my cards on the table both times, owned up to my part. When I asked them to own up to theirs, they both got offended and decided to cut me out of their lives. So we don't speak.
Yeah, that was a shocker. Twice in one year. Fun.
Also, in both cases, after breaking up with me they were in other relationships with women who were actively hostile to me. I definitely think that influenced their choice to cut me off rather than resolve stuff and transition to actual (not theoretical) friendship.
The mono guy had dived immediately (as in: concurrently, although he concealed it) headlong into a monogamous commitment with a single mono woman. She was very hostile to him not only having any contact with me, but also with his longtime former partner with whom he was still sharing a house at the time. I hear many freakouts ensued (his former fiancee is a friend of mine). I dodged a bullet there. I hear he's engaged again now.
The poly guy's wife was fine with him having a girlfriend as long as she was feeling secure, which was the first couple years of our relationship. But as soon as some stuff happened in her life where she felt less secure, she became increasingly possessive, cold and hostile. She absolutely refused to discuss with me any issues she had with me. It all blew up when she freaked out on him about something I'd said at a conference months earlier. This freakout happened on the very night before I flew out to visit him the first time after moving to another state. He dumped me on the phone that day, after I'd made the trip but before I even saw him. Her freakout was pretty crazy, but the real culprit there was his willingness to adopt her grudges and his unwillingness to stand up for our relationship.
So yes, seeing a partner's other current/former partners as threats is part of the social script, and a lot of people default to that rather than accept that everyone has connections with other people that matter, and that it's ultimately up to you to deal with your own insecurities. But it's definitely not just mono people who do that.
...Long way of saying: ability and willingness to truly be friends after a breakup may correlate with some of the skills people learn when having relationships that don't match the social script. But that doesn't necessarily mean that being poly make you more likely to be friends after a breakup. I think whether someone is/isn't mono is not the deciding factor. But I think they do need to be willing to pursue a positive and genuine post-breakup relationship ("aftership") despite societal expectations that former lovers should have nothing to do with each other.