I agree with your main point, that sites like StackExchange and Quora are not about discussions. From the get-go, Stack Overflow have made it very clear, through their writings and policies, that it is about answering questions, not talking.
Having said that, I disagree with your example. Oftentimes, when people ask a question, questioning their motives is a good idea. I know that when I ask a question, I often discover that I'm trying to solve the wrong problem.
A very common pattern on SO is that the accepted answer is a practical solution to the problem, and the highest-rated answer is a longer talk about whether this is something the questioner even needs to do.
Having said that, I disagree with your example. Oftentimes, when people ask a question, questioning their motives is a good idea. I know that when I ask a question, I often discover that I'm trying to solve the wrong problem.
A very common pattern on SO is that the accepted answer is a practical solution to the problem, and the highest-rated answer is a longer talk about whether this is something the questioner even needs to do.