Listen and Spot are stupid as skills anyway. Who the heck goes around practicing their listen and spot? You can't really get any better at perception. It's sort of an innate ability - you are either good at noticing things, or you aren't. And as people age (especially men), perception tends to get worse, not better.
The ability to spot things is affected directly by two factors: visibility and distraction. Your visibility is determined by cover between yourself and the target, and your own innate ability to see clearly. Distraction could be explained by a random factor (the d20 roll). Your innate perceptive ability is pretty much fixed.
The ability to hear things is affected by three factors: acoustics, distraction, and your innate ability to hear. Some people are better than others at hearing (obviously, on the extreme range, you have deaf people and those who can even hear at the very edge of the subsonic range), but for the most part, people are about the same in terms of 'ability to hear'. Distraction is the same random factor as seeing things (so there's your d20 roll), and the acoustics is sort of a hard thing to judge. In a vaccuum, sure, you could use the 10ft increment (although it also depends on the sound's volume). In a forest with lots of trees and cover, sound will bounce and ricochet, nullifying the listener's ability to pinpoint the origin of the sound (or possibly even the sound itself).
I'd like to see a baseline perception that's pretty much the same for everyone unless they have magical aid or spend talents/feats/whatever to improve it. I'd say the same for movement. Most people tend to move at about the same speed, but some people are faster (taller, more athletic, whatever) and some are slower (older, shorter, less athletic, a hampering injury, etc). Most game systems tend to do perception and movement better than standard D20; I hope WOTC recognizes this and makes some changes in 4E. I have a feeling, however, that they won't.