I have nothing against the on-checkExisting event, except that I think a on checkValid event would be more useful. I mean a checkValid (returning true for ok, and false for no ok) is better then (checkExisting with return false on OK).

Ok you might say 6 or one half a dozen of another, certainly if you really want to check if existing and handle as such the existing event is preferable. However there are other things that you want to check for, such as file type, size etc before uploading it. also checkExisting doesnt allow for information to be passed back. Maybe I want to fail the checkExisitng with a reason other then it exists.