If people are having fun, the game's being played right. Regardless of power level, game balance, or fluff restrictions, if a group is enjoying a game then it's being done right.
With that said, it's remarkably incorrect to say that they're a myth. The Tier System is a ranking of classes against each other, when one assumes that the optimization level is equal. For those interested, it can be helpful to have a guideline (even if one disagrees with specifics, I feel that the original tier rankings are reasonably appropriate) with which to rate a class's power, and more importantly, establish criteria for evaluating what's powerful within 3.5. It is an observation that individuals can use in whatever way they please; nothing more, and nothing less. But it is not a myth, and to state as such without any explanation or context is incorrect, uninformative, and unhelpful toward the OP.
The Tier System doesn't promote power gaming or optimization, though it can be used to help evaluate a more powerful choice against less powerful choices, and one can use it in that capacity if they chose. All that it does is inform individuals of how much potential power a given class has. End of the day, everyone should enjoy their games regardless of the way that they're being played - but don't bash a tool which many people find useful without explaining your position.