I am a pitching rate denier. There. I said it.
It's not that I don't feel that there is an optimal pitching rate—quite the contrary as a matter of fact—I feel very strongly that there is an optimal pitching rate. The problem is that most pitching rate calculators exceed the optimal pitching rate for fresh yeast by quite a large margin.
Many homebrewers on the internet strongly disagree with the claim of yeast laboratories that 100 billion cells is an adequate amount of yeast to pitch into 5 gallons of 12°P beer (5 million cells per milliliter). For instance, the Mr. Malty Pitching Rate Calculator (which requires Adobe Flash® to run because reasons) states that 168 billion cells are needed to ferment a 12°P ale and 337 billion(!) cells are needed to ferment a 1.048 lager. This amounts to an ale pitching rate of roughly 9 million cells per millileter and lager pitching rate of 18 million cells per milliliter. It is my belief that (in practice if not in theory) this is a large OVER-pitch of yeast that is more detrimental to beer flavor than a smaller pitch.
Most pre-yeast-book (BYB—before yeast book) brewing literature espouse a much lower pitching rate.