It really depends on who you are relative to the person with the idea.

Are you a prospective customer? I agree with you.

Are you a student? You're paying for the opportunity to listen, so you probably should.

Are you a product leader? It's your job to listen.

Furthermore, "showing" is not always feasible for an idea that is sufficiently complex or expensive to implement, and maybe impossible for ideas that are too early to fully articulate.

All to say, nothing can be reduced to overly simplistic solutions like "just use this definition of idea", or "just show me".