No it is not worth it in 90% of our cases, we monitor traffic religiously and you greatly compound your cost to chase what (according to out stats) is less than 5% of the market, when you compound that by the fact that you are not going to convert that entire 5% the numbers are dismal for adding the cost and complexity to a project.
We have found that one average that building in fallback modes adds 15% to 20% of cost and time to a project. We have also found that that money roughly equates to a mobile version of the web front end, which we see a higher market conversion percentage, in other words, that money would be better spent chasing mobile or adding features and revenue streams to an existing web app, than to chase a dwindling market of last gen technology adopters.
When a client of ours absolutely insists on providing a noscript site for that segment, we tend to opt for browser sniffing and segmenting that traffic to a completely separate site built for those clients. We have found that keeping the noscript version and the full version separate greatly reduces the maintenance cost of both.
