It is probably different where you are but what would actually happen is that it would take you at least 3 months to work out they can't do the job (they are probably not terrible just not good enough) by which time you have lost 3 months and perhaps $20K recruiters fee and now have to start the search again.

Of the people I have interviewed in the UK for "Senior .Net Developer", I would say that 75% did not meet the grade and that was with face-to-face interviews that took a lot of my time. An exam isn't perfect but if someone can't answer a reasonable spread of questions to a half-decent degree, then I don't want to hire them and it saves me hours of my time. Sorry.