[dspillett]

(I don't post answers on SO at all, but I do on some of the sibling sites (DBA.SE, SF, SU), so my experience might not be exactly the same, but...)

> questioning the poster's motive

The motive can be important. Where there are multiple explanations and/or solutions this can filter out some that are not going to be relevant or practical for their current situation. Also where what they are asking seems impossible/impractical/misguided/other it can help in providing a better solution (i.e. trying to give them what they ultimately want even if it isn't what they are specifically asking for) or give context that might quieten your gut "don't do that" reaction.

The poster's motive is one of the most important bits of information: Where were you? What did you do? Why? What were you intending to happen? What happened instead?

> "Don't do that."

Sometimes "don't do that" or "you can't do that" is the only suitable answer though (such as where what they are trying to do is impossible, or just wildly impractical, or will open up a security nightmare, or there is a built-in method so they'd be reinventing a wheel, or one of many other reasons) with the important caveat that you don't answer with just "don't do that".

Better is "don't do that because <good explanation>" or "you can't do that because <good explanation>".

Even better still, if what is being asked it actually possible even if it is strongly recommended against: "You shouldn't do that because <explanation> but if you chose to ignore this recommendation you could try <method>".