I personally have one template stylesheet which fills most of my needs. If a book has some special needs, I add it to the stylesheet in that book. So, in my stylesheets are unused elements. They don't bother me. They hardly take up space, especially since they will be zipped, and I don't think there will be a perfomance issue.
Editing is easier for me, because all my books use the same stylesheet.
So, if that option will be built in, please let it be turned on or off.