There are couple of things I hear in defense of progressive enhancement.
Javascript is off or not available. According to W3School's Javascript statistics, 95% had Javascript enabled January 2008 (2.5 years ago). The trend, based on those stats, seems to be that Javascript enabled browsers are on the rise. Heavy AJAX adoption has probably pushed users to enable even faster. To support text-only or accessibility doesn't make a lot of sense from the business perspective since they probably account for less than <1% of the traffic.
Mobile browsers won't work. I would either way create a mobile version of the site because of the limited screen size and a whole different browsing experience on the mobile devices. One could argue to use different CSS for screen readers/mobile devices, but you won't be able to crank up as much information on a mobile device no matter how tidy you make it look.
The original statement isn't even true for the newer generation mobile devices, like the Android and iPhone, which have a total of >200,000 combined daily activation rate. Besides, most Javascript eye-candy (like jQuery UI) doesn't make much sense because the website experience on the mobile device is so different (no mouse hover, etc.). I consider mobile devices to be a platform of their own that needs a customized version of the website. Another trend has been creating a "native app" version of the website/service.
Spiders won't be able to crawl. This isn't true either. It's possible to use the DOM as a data container for the Javascript (populate div/input tags with your entities). Also, Google has made efforts to make AJAX websites crawlable.
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