There are a few reasons I prefer the namespaced modifier approach to what you've described:

1) This approach is more error prone and less predictable. For a one-man job it's probably OK, but on a team it's harder to guarantee someone else isn't going to apply some styles to `.sidebar`, which will cause unwanted styles to be applied to your separator component. The namespaced approach is very intentional, so it's unlikely someone will add styles to that rule without knowing exactly what they're affecting.

2) chaining like this ups the specificity of the CSS rule, which means your other modifier/presenter classes might not work on the element anymore. In CSS specificity is everything, and keeping it nice and low is usually preferable.