Not worth keeping your brand looking right? This is not about code, this is about their brand and how they protray themselves. If people see a broken website (for any reason, be it cookies, JS, or whatever), they will think less of the brand.

Like you said, "...you compare the cost of the effort against the value added". It does not add anything to the brand, but not having a fallback takes away from it. And given Diggs current reputation, they need all they can get.

If >1% see a broken website, and >99% see them reviving a dead platform extremely quickly

It is their tradeoff to make, personally I think they are making the right one.

Honestly, how much time can including a <noscript> tag can take?

Simple as:

<noscript>Digg is awesome with javascript turned on.</noscript>

on top of their other markup. That's it. Twenty seconds tops.