Feedback can be very useful in order to let you see things you may be doing wrong in the interview process that you're unaware of. Perhaps you are emphasizing the wrong things, or failing to highlight the right things. Perhaps there are social aspects, customs, other norms that you're deviating too far from. Perhaps you go into an interview without a good knowledge of the company you're applying to, etc.

There are a million little things that can be easily adjusted to improve your chances, and feedback is the only way that you'll know which ones matter. The feedback from a single interview isn't helpful, but the feedback from many interviews lets you spot patterns.

> If you acted on the interview and you got hired, you'll be expected to continue the act probably for as long as you work there.

There is a small kernel of truth here, but this overstates it immensely. Firstly, you shouldn't ever "be someone you're not" -- but adjusting your tactics and adapting to your audience does not have to mean pretending you're somebody else. Second, interviews are performative and everyone knows it. In an interview, you're engaging in a sales presentation. Nobody expects that people will behave identically in everyday work as they did in the interview.