My two cents on the topic:

The tier system is a system of relative theoretical maximum power. One which casters blow the top off of, simply by virtue of having so many 'win' buttons at their fingertips.

A low tier class is not 'unplayable', because I've done it. However, I had to exert my optimization skills *FAR* more, even invented a couple of tricks no one else had come up with before, just to make it able to not be completely ineffective.

For example, the build Takahashi no Onisan (seen in sig) was a Test of Spite build using exclusively CW Samurai levels (the updated version had one level of Rogue, one level in Exemplar, and one level in Ronin, but still had 10 levels in CW Samurai). Was he effective? Sure was. I completely locked my opponent down.

However, it's a fragile build, because anything immune to Fear is immune to the character, because his overall damage-per-turn output is pathetic.

Even at the highest levels of optimization, inventing brand new tactics and finding previously undiscovered synergies... he's still not a powerful character, particularly not if opponents are either immune to fear or stay away from him.

When I applied the same level of optimization to a Warblade, I got a build which could deliver an arbitrary amount of damage to every opponent within line of sight. Because he received between two and three bonus attacks for every opponent he killed (Boomerang Ricochet + Snap Kick + Great Cleave, an Aptitude Weapon, and four levels in Bloodstorm Blade), and he would only miss on a natural 1 (Blood In The Water) and his damage kept increasing the more he hit (again, Blood In The Water), he simply used the Bag O' Puppies/Rats trick to start himself off with something like thirty or fourty bonus attacks to provide him with a buffer to ensure the combo would not end prematurely.

Do you see the difference here? CW Samurai can, at the HIGHEST levels of optimization, lockdown opponents not immune to fear within 30' of him. Warblade can deal an arbitrary amount of damage to every opponent within line of sight.

Now let's look at another paradigm shift...

At level 9, a Wizard is functionally immortal by using Lesser Planar Binding on a Nightmare to Astral Projection himself, meaning he's not even able to be targeted in combat!

By level 20, using Spell Matrix, Spellsurge, the spell Celerity, Arcane Thesis, and several other Mailman-esque Metamagic Optimization tricks, he has effectively an arbitrary number of turns available to him.

Think about that. He has every turn in the game before you have a chance to react. Yes, even in a Surprise round (which isn't surprising to him, either through Foresight or Contingency).

Can you not see the power level disparity here?

Tier 6, at the highest level of optimization, can be built to be a one-trick pony. A Tier 3 character, at the highest levels of optimization, have the potential to break a game. A Tier 1 character, however, can not only break the game, but be invincible while doing it, and can break the game in multiple independent ways.

It's not saying you *HAVE* to. It's saying these are the theoretical limits of what the chassis is capable of.