One thing to note is that while Chrome does an okay job it still leaks data between the user accounts so using the guest account servers will still know that it is the same instance of Chrome that has the main account running (aka google knows that you are in "guest" mode, but it is still you).
Except that one of the best reasons to use guest mode while you have a logged in session running is to let someone else use your computer without being affected by our polluting your logged in session. So sure, Google might know it's the same instance of Chrome but that's not the same as knowing it's you.
Well currently there is a bug where yes sites can know it is you. I have an accounts on a site and when I use guest mode that site knows it is me and auto logs me in.