Just want to add to this that not all websites are the same. In my case for example, I am building a site where there are unlikely to be any commonalities between pages, as it is closer to art than business. I still however wish to host everything off the same domain. It is very unhelpful for me to have the same CSS file for everything, as I do still like to use some common descriptors.
I would at least like to see something that gives you a more prominent alert when you first reuse a class on a different page, as when I am working fast I don’t necessarily notice the ‘1 on other pages’ line, and this can lead to me breaking things unintentionally. I’ve got to the point of having to prefix every class with the page name to avoid mistakes.
Ideally, I agree that it would be preferable to have a choice about whether or not you want to use the same CSS, just as you would if you were coding.