It's perfectly reasonable...
Then I read the fic and spend the whole time wondering why Donna is being such a useless, simpering moron. But I just shake my head and say, "damn, crazy out-of-character writing. Or maybe there's an awesome twist and she's contracted some strange alien disease that gives her dissociative identity disorder...wow, now that I think about it that would be really cool, maybe this fic actually rocks!"
And when I finish the story I'm always disappointed that there was no tricky twist with dissociative identity disorder, and "Donna" was just being a twat.
This means that you, yes I'm looking at YOU, need to write some Donna/Doc fic so that I don't have to do this anymore.