From a practical standpoint, usage seems to be the only way to tier pokemon. I mean, most other tiered games are either fighting or strategy games where there aren't nearly as many characters or 'civilizations' from which to choose. This allows in-depth analyses of each possible matchup, therefore allowing somewhat empirical determination of tiers based on power. Pokemon, on the other hand, just has far too many creatures, and therefore far too many possible matchups and situations, to make an exhaustive study of each pokemon's power. Besides, usage often dictates tiering even in games tiered by power, where better matchups against more oft-used characters contribute more to a character's viability than matchups against less-used characters. Also, usage is very often correlated with power-based tiers, at least in the higher tiers (lower tiers are always harder to determine because, heh, nobody ever uses those clowns), so usage-based tiers are likely to be in line with power-based tiers, anyway.
So, in summation, usage is a sufficient tiering mechanism for pokemon, with power being a good indicator for the banlist in the OU and UU metas (power is more feasible a criterion for tiering the short list of pokemon that might warrant a ban in each meta.)