This is a really good question.

If we remember that the writer also wants to improve his or her level of English, then I don't see anything wrong with adding a "better" phrase instead of basic error correction. Some members opt for basic corrections and others prefer to suggest a phrase a native speaker would use. If you're offering suggestions, I'd assume you've gauged the writers level and would offer something that is understandable.

I think it's helpful to see both "styles" of correction by different members, so the writer can see both the technical corrections as well as a native phrasing.  Sometimes I correct a text just to give different suggestions, but I point this fact out.

The other side of the coin is managing a translation when you know what the original text looks like - something I'm dealing with now. Some of the sentences I have to proofread are awkward and poor, and my translator assures me that the badly-written English sentence is also badly-written in the original language. So the translator has actually done a very good job, however the next step is to make the sentence "readable" without actually "improving" it, if that makes any sense. 

Stephen, this is both the hardest and the saddest question i have regarding the wople italki correction practice. There's always a 'realm of creativity' within the language. People invade it even at the beginner level, both intentionally and not. 

Moreover, some of (supposedly) unintentional uses of words/expressions/metaphors etc are just either brilliant, appealing (as a sort of personal style) or exact. 

But then somebody destroys it because he heard nothing like this from a mean Russian schoolgirl.

On the contrary, a person should be (ideally) informed on the status of every (i said 'ideally':)) phrase (s)he uses. For this purpose I wish there exist a common notion for something like 'area of creativity'... or at least for usual/unusual scale (apart from 'correct/incorrect', awkward or not, etc.):(