The Kinsey scale is a measurement that people use to label themselves -- and to mislabel others. It may be a perfectly accurate measurement of behavior, and it may have worked when the model of psychology was pure behaviorism. But these days, behavior and sexual orientation are seen much more correctly as related but different things. It was a valuable and useful thing when it first came out, because it was one of the first attempts to show that there are plenty of ways to be human other than strict heterosexuality. My argument, though, is that it's time to file it away with Freud's id/ego/superego as a first, fumbling attempt at quantification that has led to more useful things. It's a paradigm emeritus, still entitled to audit classes and invited to comment on papers, but its days of teaching classrooms full of freshmen are behind it.
For bisexuals, the Kinsey scale limits rather than frees us. One of the charges frequently leveled at bisexuals (especially male-identified bisexuals) is that we don't exist. I've seen Kinsey used in a couple of different ways to do this. One is the idea that "bisexual" is a word that describes only an exact 50/50 attraction: "Everyone has a preference one way or another, so you're either mostly straight or mostly gay." Another is rooted directly in the idea that we have to somehow "prove out" our bisexuality by having exactly the same number of sexual encounters with people both genders, or that "only behavior with both genders in the last month/year/decade counts" (leaving aside, for now, the fallacy that there are only two genders and that they are completely exclusive and opposite).
Kinsey describes behavior. It will inevitably be pointed out that "behavior is all that matters." The people who historically have been the ones pointing this out are either epidemiologists to whom bisexuality is nothing more than a vector for STDs, or people who do not wish to identify themselves or others as bisexuals, for whatever reason. But a person who identifies as gay or as straight is under no onus to prove their sexual orientation through their behavior -- or are they? A lot of straight men in particular spend a lot of time and energy trying to prove that they are 100-oercent straight (and many of them may be protesting too much). I can't speak to whether gay and lesbian people are under pressure to "prove out"; it's not a part of my experience, so I would be glad to hear your stories.
The one-dimensional nature of the Kinsey scale leads to an idea that the more attraction you have toward your own gender, the less you have toward the "opposite" gender, and vice versa. This flaw, this idea that attraction is measured in neat percentages that add up to 100, just doesn't hold up in the real world. Yes, behavior can be measured in this kind of objective and numerically neat way, but attraction cannot.
It leads to the idea that bisexuals are not wholly integrated people, that we are composed of a "straight side" and a "gay side" at war with each other, that there is an unresolvable conflict in our attractions and in ourselves that means we are undeveloped and unstable. It's one of the reasons people assume that anyone who loves us is doomed to be dumped or cheated on, that we cannot be satisfied with our partners.
Now, even if you want to buy some modification of the Kinsey scale (like the Klein grid) that includes attraction as well as behavior, it's still got problems. One of my friends has put forth a beautiful illustration of one of the basic flaws of the Kinsey scale. With hir permission, I'll restate hir argument here (modified a bit).
Imagine a pretty standard map of the world, one with L.A. to the left and Tokyo to the right. Now go to the whole population of the world and ask them, "Where do you live? Do you live in L.A. or in Tokyo? Which of these cities do you spend the most time in? Are you mostly Angelino, sometimes Angelino, equally Angelino and Tokyoan, etc.?"
You're going to end up with a huge group of people who say, "What? I live in Cairo [or Lagos or London or Delhi or New York]! it's downright silly to act as if L.A. and Tokyo are the only places that people live!" A similar problem is inherent in the Kinsey, where the "middle" of the map is not labeled "bisexual" but "50-percent homosexual and 50-percent heterosexual" -- not whole and integrated, not 100-percent anything, but half this and half that.
I'm not half anything. I'm completely attracted to the people of my own gender that I am attracted to. I am completely attracted to the people of other genders that I am attracted to. I'm completely attracted to all the people I am attracted to. Not 20 percent, not 50 percent, not 80 percent.
So where does the idea that I need to prove it, that I need to offer objective evidence to other people that I am, indeed, attracted to a broad variety of people, come from?
I don't know. I think that the reliance on the Kinsey scale in just about every popular article on bisexuality might be a part of it -- not the whole thing, but a part. There are other factors in play, as well, such as the confusion among people whose attractions are to only one gender regarding those of us whose attraction spectrums cast a broader net, and the all-too-common idea that bisexuality is an immature and unstable state, or a farce for the benefit of masculine sexuality.
But I tell you this: Every time I read an article about bisexuality and the Kinsey scale comes into it, I see red, because I know that I'm about to be misrepresented as a divided person rather than an integrated one.