I think that it is all very relative and it really depends on a child/student whose work you are editing.
I do not edit my kids school work, but if I have time I will proofread it. Sometimes they will not want me to read it at all, which is fine.
When it comes to DS, who is a good writer and writing on almost any subject comes easily to him, editing can in my opinion teach him TONS. But like Cym has mentioned earlier, the teachers do not have time, or there are other reasons for not doing it. I strongly believe that you learn by the sheer process of writing, but you also learn when someone edits your text, with explantions as to why. This is very, very important - to show why something should be written differently. So DS would benefit greately from having a regular editor of his work. Mind you, there is always this question of style, so it can be tricky :-)
DD on the other hand, is a great fiction writer, her imagination is endless, but she is having problems writing on a given subject. Because of that editing is pointless, she needs to be directed, like Kriston writes, with questions like "what are you trying to say here", "this is not clear, what do you mean" etc. She would learn a lot from a process like that. This is partially the reason she is working with writeguide.
I believe that all the important papers my kids write should be edited by someone - teacher or parent.
Just the other day when DS was putting together his science fair diplay I was editing the procedure part - it had to be re-written several times, each time with less and less scientific terms, so that a layman could understand it, and not only an aeronautical engineer:-) This is a learning process and I want to be there for my kids. I think that they deserve it.
On the other hand, like I indicated earlier, if they say no I have to respect it:-)
As for the dentist's wife, I do not think we are getting the whole picture, but if she truly cited the kid's homework as a reason for not going on vacation, that marriage is in deep trouble.
