Menshay, now I'm not saying this is true, but let's say, for the sake of argument, that no one uses Mawile this month. Because they assume it's crap. Now let's say that next month, some previously minor threat rises and starts getting crazy high usage. And then someone realizes, "holy crap--Mawile has the BEST TYPING and COMPLETELY walls this threat." Suddenly Mawile's viable again. Clearly not all the way back to the level where Mega-Mawile was, but high enough to be OU.
Well, under what you're saying, Mawile would already be in UU, since it got crap usage in September.
Now, you might say, so what? It spends two months having high enough usage in both tiers. Come December, it's back to OU. Well, here's the problem: people don't consult viability rankings when constructing teams--they consult the tier lists (this is largely Showdown's fault, but it's the lesser of two evils that things are sorted that way). So even if non-Mega Mawile is absurdly the best thing ever, it doesn't guarantee it'll return to OU, because people just naturally assume it sucks (because we put it in UU). This kind of thing happens all the time, though usually it's the other way around--drops that
should happen don't because people see the Pokemon in their tier list and assume it's good to use.
My point is that labels matter--and so we should always tread with caution, lest we create self-fulfilling prophecies (by declaring Mawile to be UU or NU, it will become such, whether it deserves to be or not).