I think we should, because just that sort of questions made SO very attractive. Rethinking and rewriting someone's text is a good example both for wanderer and teacher.
I think that we should treat this like it has been done on StackOverflow where people are assisted if they have demonstrated some effort on their part. I think it's wrong to become elitist about dishing out assistance. That said, we certainly shouldn't be doing big swathes of work for people like editing their entire book - but if there's a difficulty in a certain section and there is a specific question, I see no reason to deny an answer.
I've already stated in other questions, but I do believe copy-editing someone else's work should be something we do not do here. Tips for copy-editing should be fine, but just like with homework questions on SO, we shouldn't do the writer's job for them.