Functioning labels are wonderful. They are very convenient. Well, not for autistics; but they're great for anybody who has to "deal with" us. They work great for the typical Autism-Speaks style propaganda--"You can speak; that means you're too mild for what you say to count. You can't speak; therefore we have to speak for you." It takes power out of the hands of autistic people. Naturally, the people who are still very clearly very autistic, who have learned to speak/write/type/sign, are a little bit threatening to those who would prefer to keep autistic people out of the decision-making.
They also ignore that there are not two distinct categories of autism; that people who don't need much help can be severely impaired in some areas, and that usually severely impaired people can be highly capable in some areas; that there are those floating somewhere in the middle who can do some things, but need intensive assistance in other things; that people whose parents were told, "S/he'll never..." over and over, are now on their own and doing just fine; that other people whose parents told them, "You're smart and you're not trying hard enough," are still unable to live on their own.
When you're trying to keep autistic people quiet and out of the way, you can label them high or low functioning at your convenience. If you want to say they don't know what "you're going through" and how horrible your life is, then label them high-functioning. If you want to say they aren't competent to understand the world or make their own decisions, then label them low-functioning. And if you want to turn them against each other, label some arbitrary group one way and another arbitrary group the other way, and watch them argue over who's "truly autistic" or who is "really just quirky and not disabled at all". Divide and conquer.