Raising in tiers due to usage shows that the Pokemon's power in the higher tier is sufficient for it to be raised, which not the same as being broken in the lower tier at all. It's the opposite of the classic "<poke> sucks in Ubers, move it down" suggestion brought up by newer players. Pokemon move up into banlists if they are too strong for the tier they are in, and they move up into usage tiers if they are strong enough to fend for themselves in the higher tier. Applying this to your Gastrodon example, Gastrodon has now proved itself powerful enough to be viable in OU (as measured by OU usage statistics), and therefore cannot be classed in a lower tier than OU. I should have been more specific. Power is not uniform across tiers and can only be measured relative to the rest of the threats available (the most stark example I can think of is Sheddy in gen 4. For some time, sheddy was quite deadly in Ubers despite being utterly useless in all the tiers below.), and because of this these there will always be Pokémon in a higher usage tier which would not break the metagame below them if they were to drop. However, usage in a particular tier is a fairly good measure of power within that tier, and since one of the primary aims of the tiering system is to ensure diversity of choice it makes perfect sense to construct lower tiers around Pokémon which are less effective in existing tiers. Viable, perhaps, but less viable than those used more in OU (on the whole, because of course players don't always use the best available for their teams and new sets will continue to arise). Usage in a tier is not a perfect measure of power within that tier, but so far no one has been able to come up with another sensible non-arbitrary system.