Old editor implementations like TinyMCE or FCKEDitor had an UI allowing user to hide WYSIWYG on certain fields. Currently, this module just enables WYSIWYG for all fields having Input format with defined editor.
There should be an UI allowing user to hide on specific fields. I believe API is already done (#wysiwyg in forms API?).
We might want to create own UI implementation or get inspiration from TinyMCE, which showed specific field ID on forms and allowed to specify them as clear text