Agreed — I've always admired how extensive Sass/LESS have become, but I do think having advanced logic inside your stylesheets makes no sense. Not necessarily because of separation of concerns, but because Javascript does a much better job at this (and already has the remaining logic of the application), so there's really no need to reinvent a slightly less functional version of the wheel here.
That said, the main advantage for writing CSS in JS is being able to rewrite styles in runtime, where changing values in Javascript automatically reflects in style changes. The most direct way to do this today is setting variables as data-attributes in a DOM node and targeting specific true/false values for that attribute in your stylesheet.
Native CSS Variables will make life much easier in this respect, but it will still take a couple of years before all major browsers implement it.
That said, the main advantage for writing CSS in JS is being able to rewrite styles in runtime, where changing values in Javascript automatically reflects in style changes. The most direct way to do this today is setting variables as data-attributes in a DOM node and targeting specific true/false values for that attribute in your stylesheet.
Native CSS Variables will make life much easier in this respect, but it will still take a couple of years before all major browsers implement it.