Very little. However, if companies are making their own problems and the same set aren't used for every job posting, it would be harder to make a general how-to for every problem on the site. It also would probably not be that hard to determine which problems have publicly available solutions, or if the solution an applicant uses is publicly available - for that matter, if I were an employee of this company I might advocate for creating fake solution websites (with flawed solutions), so that applicants who were simply copying code could be easily detected.
