When you answer someone's question, you are trying to help them. If you feel that the best help you can offer is to question the original assumptions of the question, then go right ahead. Very often that can be a big help, because people sometimes get caught up in their original assumptions when they are trying to solve a problem, and someone else can just say 'Hey, why don't we find a different way to solve the original problem, rather than finding a way to do it the way you are trying to solve it, becauase there's a better way', and the questioner will 'see the light'.

But I do think it's important to keep in mind that sometimes people actually do have good reasons for doing something a certain way, so it's always nice to suggest that there might be an alternative way of looking at the problem in a way that 'gives some credit' to the person asking the question.

I think my point is this: the point of this forum is for people to help each other and learn from each other. Different people have different ways of doing this. If your helpful input is to suggest finding a different way to approach the problem, great! If the person doesn't benefit from your help, because s/he knows the whole picture and can't do it any other way, so there's no great loss, right? The best of both worlds might be 'You can solve your problem X as stated by doing Y. But, I question the need to do X in the first place - I think it's bad because of Z. Perhaps if you tell us the higher-level problem you are trying to solve, we can help you find a better approach.'