If somebody has multiple apparently unrelated academic degrees what
are the pros and cons of mentioning them?
Should they be included in a CV?
Yes, they should all be included. Personally, I believe that all education is a good thing - even when not directly related to your career field. And I believe that any degree earned is a significant achievement.
In the best case, they show that you have a wide variety of interests. They can show that you are a hard worker, and that you are willing to commit effort in disparate fields.
You may need to be prepared to answer if you really want to go in the direction that fits the role the company is offering or if that you would prefer to go in the direction your other major would take you. ("Do you really want to be a programmer? Or would you rather be an actor?") The interviewer may be worried that you are just marking time until the job you really want opens up.
Or if this would be your first job, you may be perceived as someone who would rather stay in school than be part of the working world. That could appear to be the case for someone who pursues one degree after another.
For me, I'd include all my education, and then be prepared to talk about why I chose those majors, and why I now want my career to go in the direction I have chosen.