This is exactly the point I was coming to make. I like the idea of Mincss, I have even considered making something like it before, but the problem is that if you are adding and removing dom elements with JS then I doubt this program would be able to correctly identify them all and would accidentally clear out CSS that your site needed.
That said, I am sure that static sites/non-JS intensive sites might be able to use this.
The other issue, is that it is all to often that user events trigger the adding, subtracting or refactoring selectors.
How does this handle these scenarios?