marilli
With you
Not really. See Gastrodon covering the Electric-weakness for the Water-monotype teams? It's kinda hard to get swept by an Electric-sweeper with a monotype water team, given that you have Gastrodon in your team. Similarly, mono-Electric teams would have Zapdos and Rotom-W to negate their shared ground-type weakness. If they have a bulky variant of Rotom-W, they're protecting themselves against a potential Landorus sweep or something like that. Steel-monotype has Heatran and Skarmory to cover the shared Fire or Ground-type weakness.
- Arguments such as "Water loses to X sweeper every time!" can be applied to almost every single type, and losing to Celebi is not as serious a problem, as, say, Dragonite, Scizor, or Terrakion.
I know typing isn't everything, but it sure counts for a lot. Not all monotype teams have to get 6-0'd by a [Insert threat with a type advantage]. Well, I guess Celebi isn't *that* common, so you certainly can get up high in the ladder even with such a weakness. It's BW after all, and choosing to have a single, fatal, yet uncommon weakness might be viable if your team can compensate by matching up well against other mons.