This is an interesting question. "What are arguments against user experience testing?" My initial response is that there are existing patterns that users are accustomed to, that if you make the controls and experience align to those patterns, you will not need to test such designs with users – they know it already and so the designs should be learnable and usable.
Which, in many cases, is absolutely right… and it's also completely wrong at the same time.
Because the baseline – those reference standards that we are supposed to leverage – is changing ALL THE TIME. For example, touch interfaces, and the appropriate design approaches to support them, are now part of the design space that UX professionals live in and this wasn't the case just three years ago. Controls don't work the same and user's expectations of how controls work change as well.
And I'm not just talking about design patterns – the attitudes of users change based on the world around them. How do users think and feel? it's different than it was a decade ago, it's far different than it was a half-century ago, and I dare say that it will be markedly different fifty years from now. We need to understand that mental landscape to create affective solutions.
I have worked with several people who think they "know" how things should work, and have learned "all that is learnable". They line up the form fields and controls and they think that usability testing is a validation step, not an exploratory one – and they're wrong. They are borderline arrogant – they are "user experience professionals" who know what people want and know how to make things work – and that is why they fail.
They don't know "the answer" anymore than I do – and any wisdom I have in my domain comes from asking questions and exploring how things can be different than they are today. And that comes from research and testing and failing and learning from that failure.
User experience testing that is just to verify that controls "work as expected" is not enough. If you do just that, you end up with a perfunctory solution that is "good enough." Testing and interviewing and researching to see and understand what people think and feel and do, and then providing solutions that improve their lives – that is what user experience design is all about.