obi
formerly david stone
If there were no tiers, all Pokemon would be acceptable. This would be equivalent to the uber ladder. In that environment, there are something like 12 "OU" Pokemon (Pokemon that make up the top 75% of usages). This is seen as not enough, so we divide the game into two tiers: uber and OU.
The uber tier is the unbalanced tier with little variance. OU is everything else. By this definition of uber (those Pokemon that are impossible to have in a balanced game, essentially), it is obvious that nothing is "too broken for ubers".
So now we have one balanced tier: OU. However, there are many Pokemon that do not fit in the resulting metagame. They are completely overpowered by certain Pokemon. So we say "Hey, we could make another set of tiers that would allow more Pokemon to compete!".
So we take the top 75% of those Pokemon in OU and those remain in the OU tier. The rest are moved down to a new tier, UU. By this view of UU, the only reason the OU tier exists is to create a ban list for UU.
Now we have three tiers: Ubers, OU, and UU. However, UU was created strictly by usage in OU. This does not necessarily create a balanced tier. To balance this new UU tier. our only option is to ban, as OU Pokemon are automatically removed from consideration for UU. These banned Pokemon are moved to BL.
By this definition of BL (those Pokemon that are impossible to have in a balanced game when the top x% of used Pokemon in the standard tier are removed, essentially), it is obvious that nothing is "too broken for BL", as long as the Pokemon isn't uber.
So now we have 4 tiers: Ubers, OU, BL, and UU. OU and UU are balanced tiers, Ubers and BL are by definition unbalanced.
"But wait! If BL is just a ban tier, what's the purpose of it? Why not just move those Pokemon up to OU? It would have the same effect on game play, right?"
Not quite. Remember that OU is defined as the top 75% of the Pokemon usages. If a Pokemon is OU, there can be no debate as to whether it is allowed in UU. By putting a Pokemon instead in BL, you are saying "Current forces in UU are not enough to counter the effect of this Pokemon effectively.". In the future, there may be a new style of play that makes a certain BL Pokemon acceptable. By merging OU and BL, it makes discussion of tiers more difficult and turns OU into "banned from BL either based on it's effect on UU or based on how much it's used in OU", which I think is more complicated than having two tiers for banning from UU.
However, some people realize that UU is still rather large. There are many Pokemon that cannot seriously compete in UU because there are still some Pokemon that are relatively powerhouses. So people repeat the process that was done in OU to create UU.
Look at UU usages. The top 75% are UU, the rest are moved down to NU. However, because NU so far was created entirely on usages in UU, it's not necessarily balanced. So we create a new tier (let's call it... Limbo) and ban those Pokemon that prove to be impossible to have in a balanced NU tier.
The reason we'd need this Limbo tier is the same reason we need the BL tier rather than just banning Pokemon to OU.
This gives us the tiers as follows (red tiers are unbalanced tiers that exist solely to ban things):
Ubers
OU
BL
UU
Limbo
NU
The uber tier is the unbalanced tier with little variance. OU is everything else. By this definition of uber (those Pokemon that are impossible to have in a balanced game, essentially), it is obvious that nothing is "too broken for ubers".
So now we have one balanced tier: OU. However, there are many Pokemon that do not fit in the resulting metagame. They are completely overpowered by certain Pokemon. So we say "Hey, we could make another set of tiers that would allow more Pokemon to compete!".
So we take the top 75% of those Pokemon in OU and those remain in the OU tier. The rest are moved down to a new tier, UU. By this view of UU, the only reason the OU tier exists is to create a ban list for UU.
Now we have three tiers: Ubers, OU, and UU. However, UU was created strictly by usage in OU. This does not necessarily create a balanced tier. To balance this new UU tier. our only option is to ban, as OU Pokemon are automatically removed from consideration for UU. These banned Pokemon are moved to BL.
By this definition of BL (those Pokemon that are impossible to have in a balanced game when the top x% of used Pokemon in the standard tier are removed, essentially), it is obvious that nothing is "too broken for BL", as long as the Pokemon isn't uber.
So now we have 4 tiers: Ubers, OU, BL, and UU. OU and UU are balanced tiers, Ubers and BL are by definition unbalanced.
"But wait! If BL is just a ban tier, what's the purpose of it? Why not just move those Pokemon up to OU? It would have the same effect on game play, right?"
Not quite. Remember that OU is defined as the top 75% of the Pokemon usages. If a Pokemon is OU, there can be no debate as to whether it is allowed in UU. By putting a Pokemon instead in BL, you are saying "Current forces in UU are not enough to counter the effect of this Pokemon effectively.". In the future, there may be a new style of play that makes a certain BL Pokemon acceptable. By merging OU and BL, it makes discussion of tiers more difficult and turns OU into "banned from BL either based on it's effect on UU or based on how much it's used in OU", which I think is more complicated than having two tiers for banning from UU.
However, some people realize that UU is still rather large. There are many Pokemon that cannot seriously compete in UU because there are still some Pokemon that are relatively powerhouses. So people repeat the process that was done in OU to create UU.
Look at UU usages. The top 75% are UU, the rest are moved down to NU. However, because NU so far was created entirely on usages in UU, it's not necessarily balanced. So we create a new tier (let's call it... Limbo) and ban those Pokemon that prove to be impossible to have in a balanced NU tier.
The reason we'd need this Limbo tier is the same reason we need the BL tier rather than just banning Pokemon to OU.
This gives us the tiers as follows (red tiers are unbalanced tiers that exist solely to ban things):
Ubers
OU
BL
UU
Limbo
NU