Well, plain old CSS is just plain old CSS. There's nothing forcing anybody to introduce a processor, pre or post, into the mix if one doesn't want it.
Sure, but if you're talking about true "plain old CSS" then there are some huge issues that are going to come up in many situations. It's not a particularly comparable solution in terms of power, whereas pre/post processors are comparable with CSS-in-JS.
What exactly are these huge issues?
CSS-in-JS just cuts out a whole swathe of tools and replaces them with what is essentially just JS. Even Radium is extremely limited, because just by inlining styles using pure JS most usecases are already covered.