First, it's a small percentage. Or believed to be a small percentage. There are some larger buckets (eg distributions that disable telemetry), but it's possible to do very rough estimates of the sizes of those through other means. And anyway, the vast majority of users are on Windows.
Second, it's fair: if you disable telemetry, you're choosing to not be considered in any telemetry-backed decision making. If you want to still be considered, then it's up to you to make your opinions heard in some other way. (Filing bugs or https://connect.mozilla.org or discussions in places like here, though note that the latter is mostly useless. Not many Mozilla people read this forum or take what is said here very seriously. And even if they do people will be vigorously arguing both sides so it's easy to pick the side you already agree with.)
There's nothing wrong with disabling telemetry. I respect the decision, and I'd certainly rather have people using Firefox with telemetry disabled than have those people not use Firefox. But it's your browser, and even the social contract by which you're using it doesn't say you owe us telemetry data.
Second, it's fair: if you disable telemetry, you're choosing to not be considered in any telemetry-backed decision making. If you want to still be considered, then it's up to you to make your opinions heard in some other way. (Filing bugs or https://connect.mozilla.org or discussions in places like here, though note that the latter is mostly useless. Not many Mozilla people read this forum or take what is said here very seriously. And even if they do people will be vigorously arguing both sides so it's easy to pick the side you already agree with.)
There's nothing wrong with disabling telemetry. I respect the decision, and I'd certainly rather have people using Firefox with telemetry disabled than have those people not use Firefox. But it's your browser, and even the social contract by which you're using it doesn't say you owe us telemetry data.