I think ELL is a special case where even trivial edits should be encouraged if they're correcting obvious grammatical errors that wouldn't be made by a competent native speaker.
Many questioners here will make minor errors (wrong verb tense, missing article, etc.) that don't detract from legibility, and would therefore often be left "as is" on other SE sites. If other non-native speakers do recognise the errors, they may (subconsciously, at least) think the worse of ELL. If they don't, the site risks causing people to acquire or retain bad habits.
Having said that, I wouldn't particularly want to encourage edits that just "improve" poor phrasing, etc. Not that I think there's necessarily anything wrong with doing that in many cases; I just don't think it should be explicitly either encouraged or discouraged any more than on other SE sites.