Personally, I don’t believe in plainness. I don’t think it exists. I’ve never seen a person who looked plain. And I refuse to allow people to be rendered invisible and meaningless by their appearances. How can you have a face and be plain?
In my interviews with women, the word itself has come up quite a bit (in the transcripts, not in what I've published), but I've noted that the word only comes up when the speaker is talking about other people. "I have friends who think they're plain," "Somebody might be what you might call plain"—that sort of thing.
You know what word hasn't come up? Ugly. Except in one distinct context: When the speaker is saying what they're not. "I mean, I know I'm not ugly or anything," "I don't think you'd look at me and say I'm ugly," and so on.
It's an interesting contrast: People like to say that women are critical of one another, but the women I've spoken to (who, granted, are self-selected) have been very hesitant to suggest that another woman could be a word that's as ugly as, well, ugly. It suggests a sort of aggression, a sort of personality disorder in addition to any lack of physical graces. Plain, on the other hand, seems somehow kinder, even though, as Kate points out, it means that "you're not even fascinatingly strange looking." But it suggests someone who may have a strong moral character, someone whose features are assembled as one would expect.
I'm not sure what to make of why women are more eager to apply it to themselves as a contrast point of what they're not. I'm guessing that it's because by setting up a contrast of how they think they look, setting up an aggressive standard like ugly sounds less conceited than something more neutral like plain. Saying you're "not ugly" could mean that you conceive of yourself as being plain, or as being pretty or beautiful or striking or whatever—but it leaves it up to the listener to think on it, without being specific, which is interesting because ugly is a pretty specific word.
"Not plain" means that you acknowledge that you're—I dunno, decorated? The very notion of the word plain and how it's used in older books is more of someone who hasn't been graced with the features of what our society traditionally considered beautiful—that you haven't been "decorated" not by your own hand, but by the powers that be.
Clearly a certain art director never read the book.
I'll be honest: I dislike the word too, for the reasons Kate enumerates, but honestly I'd rather be called plain than ugly. It fits into that whole notion of thinking of myself as low-key, leaving whatever beauty someone might find in me up to the other person, not my own actual features or what I've done with them. And let's not forget that there's a whole subsection of romance novels devoted to "plain" heroines. There's something deeply appealing about the notion that "plain" women, by virtue of their charm, charisma, kindness, vitality, or other virtues, can become beautiful—physically beautiful, not only full of what we refer to as "inner beauty." We want to champion that heroine, and we feel more virtuous in doing so, because we're all so attached to beauty that we feel as though we're automatically rooting for the underdog.