The thing about 00bers is that they're powerful, but too powerful compared to catchable-in-multiples-per-playthrough Pokémon. Kyogre, a Lati with Soul Dew, Rekkukuza, Kek-king, etc are all perfectly beatable, but not by the average Pokémon.
I'll give an example. Take this behemoth:
Slaking @ Choice Band
Nature: Jolly or Timid
EVs: 252 Atk, 252 Spd, either 6 HP or 6 SpDef
Hyper Beam
Earthquake
Shadow Ball
Brick Break/something else
Sure, a Trapinch or Dugtrio with Protect could make that thing cry easily, but... for Trapinch or Dugtrio to beat Kek-king, they have to switch in safely. This doesn't solve the fact that Kek-king can OHKO anything in 1 turn. The previous Pokemon you had out has to be sacrificed to OHKO... only then can you bring out Trapinch and Dugtrio and beat Kek-king for free guaranteed. This isn't preferable at all.
Well, there IS another slightly easier way to do it, which will spell doom for a few other guys on their team as well... "Hi, my name is Heracross, I have Endure, Reversal, and a Salac Berry, and I'm gonna beat you up!" I shouldn't have to say the problems inherent in that, however.
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This entire post has been about how ONE simple uber has a hard time being beaten... multiply the headachy-ness by six for a full team of 00bers and you'll get why people tend to ban them.
