Example edit here: https://stackoverflow.com/suggested-edits/18199
It is possible that this was the same guy actually making a correction, but we should require them to log in first.
Considering that about 30% of the edits are approved, I think anonymous users seems to be very helpful in editing posts and therefore see no reason to ban them from suggesting edits. This is almost one out of three and is a bigger number than I expected.
Sure, one could argue that it should be at least 50% for it to be acceptable, but if you consider the effort required in making an approved edit (some edits might take half a minute, some edits might take several minutes) vs. the effort required to decline a bad one (normally you can spot a bad edit very quickly), I'd say that 30% is a very good number.
I also think that allowing anonymous users to edit posts is a sign of openness, a way to say "Welcome, please come and improve our site!". Disabling this possibility would be like saying "We surrender! You destroy us too much with your useless edit suggestions!", which is not something I am willing to say.
Edit: Another thing. I don't think that we should punish all anonymous users just because some of them can't behave. Have you ever been in a group where someone did something bad an the whole group (including you) were punished just because some people misbehaved? (Hint hint: Pre-school / Kindergarten perhaps?). In situations were the group is gathered together this method makes some sense (although I still don't like it even then), but our anonymous users aren't gathered; they can't communicate with another, the good ones can't influence the bad ones' behavior. It would only be unfair to the good ones to punish the whole group.
Most suggested edits from anonymous users fall into one of three categories, which I believe I'm listing from most common to least common:
Preventing suggested edits from anonymous users would raise the barrier for drive-by experts to contribute corrections. We should encourage these drive-by experts. Don't hope that by forcing them to register, they would stick around — if you force them to register, most of them will pass by and never contribute.
Barring anonymous users from suggesting edits would not solve any vandalism problem: vandals don't hesitate to create hundreds of accounts to post spam, obscenities and other unwelcome content. The only problem that preventing anonymous suggested edits would solve (or rather, significantly alleviate) is attempts to reply. It is this which has to be balanced with the loss of helpful corrections. And that would be throwing out the baby away with the bathwater.
I just rejected this vandalism edit and can't for the life of me figure our why we'd allow anonymous users to make suggested edits. Let's restrict that ability to those who are signed in.
The success rates (over all time recorded in SEDE) based on anonymous vs signed-in users is