bumpski - 1:33 pm on Apr 6, 2012 (gmt 0)
Google wants to know what your page looks like to the user. If you hide your css and/or js, Googlebot can't see that.
Google IP addresses must be accessing almost all css and js files now. Google's "preview" is likely a foundation for Panda. It would explain the slow update rate of Panda; it must take a lot of horse power to generate all those previews. With Preview they can parse the Document Object Model (the structure) of all your pages, which means Google can fully understand your page's content structure, js, and css now.
The only thing left for Google to find out is how a site might be serving pages differently via js and css to the many browser types that are out there now, probably especially mobile. They probably can't afford the horse power to render "previews" for all possible browsers.
So with preview they've already reviewed your css and js files, now they just want to see the variations.
Given that context, it seems less likely Google has many evil intentions by asking webmasters to unblock the few remaining js and css files Google hasn't already reviewed, and fully understood, by parsing the Document Object Model using the "preview" results.
In the past I had one comment to a post about CSS; that being "how could Google possibly understand CSS?". CSS must be a lot easier to understand than English, Spanish, Japanese, etc; which is Google Search's goal.