Each on site analysis provides a pretty good reason as to why that Pokemon is placed in the tier it is in the form of describing the specific strengths and weaknesses of that Pokemon. If you want to get an in depth reason, you can ask it in the respective Simple Question, Simple Answer thread stickied at the top of each Smogon Metagame forum. If you have a specific Pokemon in mind, you're more than welcome to discuss it in the forum for the tier its currently placed.
Basing tiers off if usage seems pretty intelligent to me. What else would you suggest?
Also I agree with everything TheMantyke said. He is a wise man.