With respect to the edit suffix.
It doesn't completely nullify (parts of) comments / answers that may still be useful, or cause undeserved down-votes because of answering a question that's no longer asked, but still being a good answer to the original question (though this kind of change is only acceptable to a minor extent in any way) (which could happen if you rewrite the post).
It more than rarely happens for (mostly new) users to modify their questions to address completely a different issue, which resulted from applying the fixes specified in the answers. While this practice should probably be forbidden, while it isn't, it probably makes sense to add an edit suffix rather than getting rid of the original question and thus invalidating applicable answers.
It shows a sequential flow of what happened (to some extent). Sometimes this is significant, sometimes this is rather pointless information. There may be comments / answers and then edits made in response to these.
For this, it might be a good idea for a moderator who sees this, under certain circumstances, to change the post and delete the comments which were replied to via edit (or the poster can flag the comments as obsolete, but these flags may get declined).
People who have already read your post are able to instantly tell what you changed.
You can look at the revision history, but there are often cases where you make changes that aren't particularly easy to diff, and the edit summary is not exactly sufficient to explain all changes sometimes.
It's easier to know something (significant) has been changed. Quite often, edits are just tiny fixes that doesn't really add much for someone who's already read the post. Looking at the revision history for each of these seems like overkill.
It's less effort for the editor.
The post is a probably bit longer.
The post might be a bit less logically organized.
You may read the entire post before coming to an edit which invalidates a lot of it (in this case an edit suffix should probably not be used).
In my opinion:
EDIT: Fixed syntax error
EDIT: Changed various things
An edit long after the post (except for updating broken links, fixing syntax errors, etc). Not sure how long "long" is.
Something that may actually just be additional information.
Fundamental question changes (as above) (whether or not they should be allowed is a different story).
EDIT: It may be faster to use merge-sort rather than insertion-sort as I did above.
"EDIT:" is probably not required above, but under certain circumstances...)
UPDATE: As of MyOracleServerSQLFirebirdAccessLite version 487954.22, this has been fixed.
Under certain circumstances, a (temporary) "Edited" comment may be a better idea (specifically in response to someone's comment noting an error or similar).
Sometimes it's applicable to put the "EDIT" or "UPDATE" at the top of the post, which instantly shows the change as you start reading.
There are good points and bad points. The edit suffix should definitely not be blindly applied to all edits one does, but it does appear to have its place.
Except for fundamental question changes and updates like the one shown above, it's probably better to incorporate the edit into the post rather than using "EDIT", but I'll probably keep doing it occasionally until functionality is implemented to prevent it.
EDIT: I changed some stuff, can you find them, or do you need to look at the revision history?