One of the best current reasons to have a fall back is performance. It turns out that 100% JavaScript web sites have a horrible performance profile. Having just got back from Velocity conference I can say half the performance talks were about postponing as much JavaScript on the page as possible to cause faster render times.
Accessibility used to be a decent argument (at least to those of us who cared) but it isn't any more. I say that as a co-author of WCAG2.
Would you expand a bit on the accessibility aspects of this? One of the only reasons I even worry about using Javascript heavily in my sites these days is due to accessibility concerns -- are you saying that folks who are blind are able to use Javascript now?
There have been a few JavaScript accessibility arguments over the years. The main thrust goes like this:
1a) Screen readers (e.g. JAWS) don't support JavaScript properly
1b) Only very modern screen readers support JavaScript properly (JAWS 10+)
2) Upgrading screen readers is expensive
3) People with disabilities statistically have lower incomes
QED we can't expect disabled users to have JavaScript.
To address the points:
1a was true a long time ago, but modern screen readers do support JavaScript and things like ARIA.
1b is somewhat true but that proves it's a technology rather than an "accessibility" problem, the screen readers weren't implementing JavaScript rather than it being an actually accessibility issue. As such WCAG2 made the decision to treat it as such.
2 and 3 are somewhat true, however many people with disabilities have access to non-profit, state or federal programs that support them with technology. Also free, open source projects like NVDA are increasingly advanced.
Accessibility used to be a decent argument (at least to those of us who cared) but it isn't any more. I say that as a co-author of WCAG2.