Most clients display images and most images are just web URLs.
So, while the text of notes cannot be modified, images absolutely can (by just replacing the resource at that URL. No need to modify the URL itself).

This isn't necessarily useful for advertising, but can be used for many of the evil goals that editing can be used for.

The main issue with editing, in my view, is the fact that it makes rendering every post inefficient: currently, without edits, the client can display a post as soon as it fetches it, then cancel the request for that post towards all relays. With edits, the client needs to fetch all edits and be sure to wait long enough, since the first answer it receives might not contain the most recent edit (that's because relays have incomplete information).