Well, let's see. If I weren't friends with any exes...I'd be missing out on several greatly treasured friendships. And if my significant other had a jealousy issue over a platonic friendship of mine, that'd raise a serious alarm bell for me.
Don't get me wrong - I'm not saying people must be friends with their exes, or even that they ought to (you know, in a prescriptive way) try to be. And I get that some people don't want to be friends with any of their exes. I get that some people can't be friends with some of their exes (I do have exes I'm NOT friends with, too). It's just the black-and-white proclamations that people make that are ridiculous. And the assumption that friendships with exes are inherently threatening to a relationship.