I forsee a problem, or at least I think I do.
Say Pokemon X (let's say Shedinja, because this holds true for it) is almost completely unusable. Tier 1, 2, 3... it can barely hold its own in any of those. But it's used a bunch in tier 8 or 9, because there are so few counters for it there, and it has a small chance to shine- what happens then? Is it promoted to tier 9?
And also, on the whole 'fair share'- I'm wondering how that will be affected by the fact that, say in tier 7, there's a Pokemon that is usuable, but it's in tier 6. It isn't overwhelming in either tier (why it's in 6), but it's there. How would this (or, realistically, many pokemon like this) affect the tiers?