You seem to be missing the underlying point.
It is the same editor used in the ACP as the front end. If you edit the bbcode library, it will not only affect the acp, it affects the front end as well. It is the same code. One file. Used in both ACP and front end.
If you edit the file to remove the rules that strip out javascript, users will be able to submit the content unfiltered when making posts. Now, if they don't have HTML permissions they can't create the full tags of course, but that's not to stop them from injecting XSS via onerror, onclick, etc. event handlers which WERE checked for and stripped, but no longer are since you removed that code.
If you are determined to do this, however, no one can stop you - but you need to post in the resources forums. This is no longer feedback for IPB. :)