I just can't imagine a judge being okay with a physician witness getting up on the stand and evading questions they do in fact have the capability of answering after everyone has gone through all of the rigamarole of assembling a bunch of attorneys and jurors. I'm sure the law varies by jurisdiction significantly on what exactly the judge can do in that situation, but I'd certainly be peeved as juror. Maybe the reality is that it works to scare off the attorney from subpoenaing you as you might become hostile to their case? And of course the lawyers can't throw you in jail by themselves, they'd need the also peeved off judge to do that once you actually got on the stand and declined to answer questions asked. If a physician says they did an evaluation, the attorney most certainly will ask what the results of that evaluation was. If the physician answers with, "I won't tell you, I haven't been paid for that," I don't see it going well.