Its not the editor per-say, but the functionality it unleashes. People that are even mildly versed in how the php server/client funcionality works and has a basic understanding of this code (by downloading it as all of us have) could easily create their own page that would allow them to upload whatever they wished, that could very well compromise the currently existing html on a page. By restricting things to board bbcode only, it allows it to directly interact with the currently existing forum, without needing to overwrite/change currently existing post code. Plus, if there is a way to uplaod raw html at all, even if it is first parsed through something client-side, people will find a way to do it and use it.