Native Speakers and British Exams

Yes, of course a native speaker of, say, German, is going to do well at least in the language, content of an German oral exam. You would expect them to be fluent, to use tenses correctly, show good understanding of the grammar and use correct intonation and pronunciation. If they speak clearly and remember to show their knowledge of their own language.
However, even if they do slow down so that they speak clearly and remember to add in quite a few conjunctions which change word order (German again), they still may not get full or even high marks, especially in AS or A2.
They still need to know how to structure an argument, to develop it and to sustain an idea through debate. These are more pure language skills than just speaking in a given language. Even in GCSE, where the presentation of a topic and the consequent discussion will be on a much simpler level, just being able to speak fluently in the foreign language may not be enough to get the very top marks. That two minute speech should be given in a way which makes the topic interesting and allows for a meaningful discussion afterwards. Also, in GCSE, the second role play in the Higher exam is actually a narrative piece, and those performing this will need to know a little about narrative structure to obtain the highest grades.
At first, this may beg the question of whether the exam really is checking performance in Modern Foreign Languages. I’d argue that the exercises described above are actually extremely appropriate. Language, one’s own or one that you learn later, is all about communication, and these exercises test whether you can communicate with the language as well as merely use it. When we look back at the Communicative Language Initiative, the emphasis here was on communications – sometimes at the expense of grammar. There is, in fact, a place for all these components in language learning.
Perhaps the question we really need to ask about our language exams is whether they really test what we really need the examinees to be able to do. I rather suspect that we are simply allowing an academic tradition to bend towards something more meaningful rather than cutting straight to the chase.