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Consider GMail. How awesome would it be if you didn't have to wait for its "Loading..." progress bar? It takes 2-4 seconds for that to go away on my computer. If you design for progressive enhancement, you can interact immediately and when JS is ready, it simply enhances what's already there. If done right, it's all the same interface.
If you don't mind waiting, your users may not agree: Google and Amazon both have noted significant losses in revenue at page delays much shorter than 2 seconds.