I think it depends on the quality of the question. For instance, if it's a semi-vague question just pointing in the direction of a solution, then I think it's

I think it depends on the quality of the question. For instance, if it's a semi-vague question just pointing in the direction of a solution, then I think it's fair to question the motivation of the poster. But if it's a very precise question about how to accomplish something specific, I think the burden should be on the person answering to add the clause: "you can do X, but its generally not considered a good idea because Y". Honestly if you can't answer the original question, I get the impression you're just trying to be a smarty-pants and show off some domain knowledge you have which is related to the task at hand.