That option is already available via the Modify ePub plugin. However unless you plan on reading your books using calibre's ebook viewer, there really is not much of a need for it for anyone else - if you use the "proper" channels of send to device/save to disk for reading the book on your device then the book metadata will get updated that way.
I agree it can be a little confusing at times if you are using the calibre ebook viewer. When I first used calibre I couldn't understand why when I renamed the title of a book in calibre the ebook viewer was showing the "old" one, and likewise with covers. It is one of those things that "once you know" the why and how to work around it you forget about it, and if it still really bugs you then either the Modify ePub plugin or doing a same format conversion will fix it.