I'm not defending whiteboard interviews, but I don't think it's a secret very very few candidates answer the questions from real experience. Rather, it's obvious the main approach to dealing with whiteboard questions is to read up and memorize the stuff before an interview.

To what extent are companies using whiteboards to test who is serious about getting the job? And who is willing to put in extra time and occasionally do unpleasant extra work?

Again not defending whiteboards, and I can think of ways to improve it especially for the more experienced engineers. But all else being equal in terms of two candidates experience and skills and fit; the candidate who was able to memorize things just for the sake of the job, who did research and who came prepared etc. Well that candidate might have the upper hand when making the hiring decision.

Right or wrong, companies do want to test for things besides current programming skills, like being a team player and do things that aren't easy or enjoyable. 80/20 rule maybe applies. How else to test this stuff if companies decide it's important?