The tier system can be used as a method of choosing the mechanical elements of a class. If you consider multiclassing, you can get a very general idea that combining two classes from different tiers would affect their overall play style. This is not necessairly a bad thing. I have also used the tier system to balance myself out, and make sure I wouldn't be stepping on too many toes.

That being said, the tier system is not nearly dynamic enough to cover true Optimization. I can take enough dips and ACFs in apparently low-tier classes, and make a fairly high tier character.

Using the Tier system as an end-all reference to optimization, I would have to disagree. It could be useful in introducing new players into the general power level of the classes. This can, so long as it is clear that it is not definite, can be a useful teaching aide in character optimization.

As for the Stormwind Fallacy that tends to follow these discussion, I feel that it depends on the person. For example, I have played a masochistic blood worshipping healer that doesn't do anything but heal. The literal T5 healing class. One of the only sevierly low tier casters. Would I be terribly effective at much? Not really. But I enjoyed the concept of worshiping blood, and even planed on writing a book in character about Healing and the Life Blood, out of my own blood.

Just because you aren't contributing as much as the wizard does not mean that you can't have fun with it. It really just depends on who you are.