So.. Once you know and understand the solution to a problem or a puzzle, what's the point (with respect to the company that is hiring) in candidate getting luck

So.. Once you know and understand the solution to a problem or a puzzle, what's the point (with respect to the company that is hiring) in candidate getting lucky in the interview and answering that question? Usually people say that solution is not the important thing for a question but the process of getting to a correct or wrong solution is important. Some times interviewers ignore this fact. Instead of puzzles or problems which may surprise some candidates, how about American Idol auditions style interviews. For example, you ask a candidate (for programming job) to design/implement/test some small problem giving the full requirements or simulating changing requirements. Will this process help select a right candidate?