is it also something about longer term intentions? If you're on a hook up site and explicitly stating you are just looking for casual sex then you expect the guy to deliver what he's promised - however, if you have any view of a longer term relationship - be that friends with benefits or something more, and you're a bottom, or in my case a top, then 'dating' a versatile guy will eventually mean he wants to flip - either to be the bottom if he's been top with you or to top if he's been your bottom guy.
I know I've certainly experienced that with vers guys, I explain i'm top and always will be, and they say 'yeah that's fine I love to bottom' but then after a while they just want to fuck me, and i'm like well that's not what we signed up for - I was explicitly clear.
That said, as other posters have, the majority of vers guys I've been with have actually just wanted to bottom, enjoy it and not really want to top. (Ok smallish numbers but my experience).
So I think its more about communication and being honest from the off as to what each partner is really looking for and accepting that sometimes, no matter how hot a guy is, that he's just not right for us?