I understand the rationale behind this, but honestly, I don't agree.
There are certain (and in my business, often) instances where JS is not only required for usability, but because it simply can't be done without it. Certain tools and features, made to accomplish a specific task, require a certain level of technology before they can be used.
An example of this is gaming. Every new game seems to require an even higher level of hardware. Without this hardware, the graphics and frame rates we expect are not possible - and honestly, I wouldn't play the game on old hardware anyway, because the experience wouldn't be a positive one.
The same goes for the UI. The level of interactivity and ease-of-use that users have come to expect require certain technologies. Without them, you just can't produce that level of quality.
Obviously, there are cases when this is not true. Audience is important. If you're site is geared toward third-world countries, you need to build for that. Same with disabilities.
Why would I do that?
Simple cost/profit analysis. It doesn't make sense to spend 1/3 (at least) more of the time making it work for 1/10,000 of potential customers. It just doesn't make sense.
If you want to go 4-wheeling the Utah desert, you have to have a 4 wheel drive vehicle - not to mention bigger tires, good suspension, a roll cage - the list goes on.
I believe your statement holds true, felgall, for a public sites aimed at the general population. Or perhaps a contact form for lead generation where you don't want any chance that someone can't submit. But when you're talking about an advanced UI with enhanced functionality aimed at a specific end result - it just doesn't make sense.
(Obviously, if someone does log in and turns JS off, you need to make sure they can't do things (like submit a form) that would cause undesired or dangerous outcomes.)
PS. By the way, felgall, you're awesome. Thank you for all you contribute to this community! I just disagree with you on this point