Here from the newsletters. I agree with those saying concrit or any crit on anon stories is not appropriate, and thus won't rehash their arguments.

I would like to add, however, that readers do not necessarily know which stories were written by pinch-hitters. It's fine to go into a story and think, "Oh, this needs a bit of ironing out in places," but it's very possible that the story was written in four days, to spare the recipient from not getting anything at all!

I think that's reason enough to hold the concrit in anon exchanges, and by doing so possibly give a pinch hitter a break. :)