[rusanu]

I don't like this because it leads to the misleading assumption that it doesn't matter which value you refer (left or right), since they are equal. But once you consider collations is easy to see the fallacy:

   left.id = 'Something' COLLATE case insensitive
   right.id = 'SOMETHING' COLLATE case insensitive

   SELECT id FROM left JOIN right USING (id);

Will the value be 'Something' or 'SOMETHING'? Better avoid the surprise and be specific...