The best debaters who I knew personally really identified themselves with debate. Two of them went on to coach debate in college. The better you are at debate, the better you think you are at arguing. For them, believing that policy debate and real logical argumentation are substantially different things would imply that the thing they're good at is a mere game, rather than a pragmatic generalizable skill. Psychologically there's a powerful motivation to want policy debate to be more real.

So I think it's harmful to the degree that the debater doesn't keep in mind the artificiality of the format.

All that said, it's fun, it teaches you to think and speak on your feet, you get to socialize with other likeminded people, and you learn a thousand times more about politics and history than you would learn in any other class.