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.
