I paid for some interview prep and found it very useful as a retiring air force fighter guy. It helped me to examine my experiences and see where some of the things I've done are somewhat useful to a person interviewing airline applicants. I also hadn't interviewed for anything with an HR person before, or for any job outside the military. My lack of interview experience made me pretty nervous. Doing the prep let me interview with quite a bit more confidence, which allowed me to "be myself", which wouldn't have happened without the preparation. During the interviews, I think I gave one somewhat canned answer, but the rest were just honest conversations.

I would think someone comfortable with, or at least experienced with HR interviews, and with obviously relevant airline experience would get less out of it. My specific situation and personality lent itself to doing some guided prep work I would have struggled with doing on my own.

Just because you prepare, doesn't mean you have to act like a robot reading a script.