I have often had to justify the benefit of User Experience. I actually think it's quite simple: effective people make for effective technology and effective technology makes for a successful business. And UX is all about creating happy, effective people.
I was asked the other day about my particular design bent. Am I more inclined to simplicity or complexity? There is no right answer to that question.
It is interesting that users are capable of adapting to a lot of what we might call poor interfaces. That creates some interesting problems when we go to make improvements.
It doesn't really matter how skilled a user is when they encounter our applications. We should treat everybody the same knowing that they are going arrive at their own version of success within our application based on how well we design to good usability principles.
As Steve Krug points out, most people don't really attempt to learn an application by reading. They simply dive right in and attempt to learn by doing. This might be the most simple and basic usability theory.