Sadly, many of these sites and interstitials are driven by CSS.

Really? Do you have any examples I can point my browser at? My experience is that, yes, they're implemented in CSS, but triggered by javascript.

Occasionally, I have to inspect the page and add a "display:none;" Alternatively, if a site is totally broken without javascript and I don't want to add an exception for it, I'll open it in another browser.

This method is a bit of a pain, but it's the lesser of two evils (for me).

You're probably right that it's both. That said, you can nuke these with CSS. I've taken to pretty liberally modifying site stylesheets (I've compiled almost 700 of these), and nuking things like interstitials, "social" toolbars, "related content", slide-outs, etc., is high up my list. A few of those are now in my default CSS (the Tumbler teaser most notably). Pretty easy to do using Stylebot (Chrome).

The only time I enjoy the presence of JavaScript is during particle simulations.

...well, maybe fractal demos too.