Yes they collect dirt over time, but it takes 10 seconds to clean. On my Kensington Orbit Scroll you can lift the ball out instantly, just swipe your finger over the lights (the sensor is always clean, it’s the red leds that get dirty as they touch the ball) and you’re good to go again.
I see what you mean about using an optical surface instead of a trackball, but the momentum of a physical ball is really nice, you just give it a whirl and it’ll speed across the screen. To achieve the same thing with an optical surface you’d need some acceleration algorithm in software (like windows mouse acceleration, or what they use on trackpads), but I personally prefer a 1-to-1 mapping of my mouse and screen.