The only way that UX can be globally optimal is if all users share the same goals, the same skills and understanding, the same capabilities (browser caps, etc.).
In the real world, where people have at least slightly different goals, are more or less sophisticated, and sometimes use IE6, compromises -- sometimes deep ones -- must be made.
For example, on our ecommerce site, we've got at least two very different user personas. One is what you might think is typical -- an end-user who browses, trying to figure out what to buy. The other is a purchasing manager, simply working his way down a purchase order. In many parts of the process these two people have very different needs. Obviously we want to try to cater to both of them, but multiplying the options in itself has effect on the UX.
In the real world, where people have at least slightly different goals, are more or less sophisticated, and sometimes use IE6, compromises -- sometimes deep ones -- must be made.
For example, on our ecommerce site, we've got at least two very different user personas. One is what you might think is typical -- an end-user who browses, trying to figure out what to buy. The other is a purchasing manager, simply working his way down a purchase order. In many parts of the process these two people have very different needs. Obviously we want to try to cater to both of them, but multiplying the options in itself has effect on the UX.