I have never been asked by an interviewer to provide an expert review of the client's product/service as part of the interview process. I would assume that this is a good opportunity to see how a UX designer approaches the potential task of improving the user experience, and also a good opportunity for the UX designer to see the company's attitude and ideas about UX design. Too often I see job descriptions posted requiring UX designers to do user research when it is clearly out of scope for the project, or for UX designers to have knowledge of Agile processes when the company does not implement any. I think it is really strange to accept or reject applicants based on what is written on a resume, because I often have problems believing what is written for a company job description.
What is the main problem with just asking the applicant to review the company's product/service, at least from the perspective of a potential user?