I think forensic fellowship is worth it if any of the following apply:
-You want to do a significant amount (let's say >10% of your time) of expert witness work.
-You want a leadership position in corrections
-You want to teach in a forensic psychiatry fellowship
-You have a passion about the interface between psychiatry and the law and do not mind if you take an income hit to do the fellowship
For any of the above, the forensic fellowship is helpful but not essential. If you do want to do a significant amount of expert witness work, the fellowship will definitely help. You will have built-in experience with a range of cases, lots of practice and feedback with report writing and formulating forensic opinions, a network of people who are more likely to refer to you, and board certification. You can build those things without forensic fellowship, but I think it would be an uphill climb so if you actually want to make expert witness work a major part of your career the year is worth it (including financially).
In my experience, outside of expert witness work I think many forensic settings (like corrections or state hospitals) really don't require the fellowship. Even if you wanted to, for instance, work on a prison or state hospital ward treating sex offenders, you can probably find such a job and learn more about that one subspecialty area in a year of attending work than you would in a broader fellowship year. If you don't mind the financial hit, though, I think a forensic skillset can be helpful in a broad range of settings.