There is something to be said about the expectation when js is on and off. A lot of sites will have a notion of javascript is off == old mobile phones or low level device, and will assume that when js is off most css will be too.
Simple sites won't see this as a problem, but for mildly complex sites, you'll have a matrix of "is js on ? off ? " "is css on ? off ?" "does it accept cookies?" "is this html5 thing available ?" "is there enough memory for this or that?" , and you'll add to the mix screen sizes and the availability of a keyboard or not, etc.
There's already so many moving parts, saying "this browser handles cookies, every newest features of css, html5 and everything. Except js won't run." is just a recipe for ugly user experiences.
Slightly OT, IMO blacklisting specific js (e.g facebook, twitter etc), or having browser giving an option to kill scripts that take too much time or too many resources should be healthier for the devs and the user than just turning js off on the whole site.