This form of test, then, has positive backwashing results, in that, in order to prepare for it, students would need to practise both interactive skills and grammar. It is also a legitimate test in that, assuming that the overall teaching goal of the instructor is to improve the communicative competence of his students, the test is aligned with the objectives of the course.
FACE VALIDITY
The face validity of the test may also be questionable, particularly if students feel that their success depends on the performance of their partner. One solution will be to work with the examiner or, if possible, an instructor while the examiner is listening.
Finally, with regard to spin-off, there isn't anything that the instructor can do afterwards in the lesson, unless it was somehow documented.