Justifications include Googlebot's blindness to such content and the anecdotal user with assistive technology. (Not really hypothetical, I met some.) But even then I couldn't help feeling that perhaps their software ought to be capable of a bit more, rather than the web being limited by the most basic or old user agents.
This article from Google's Webmaster Central Blog gives their view of the matter. As well as useful tips like making sure the necessary js and css files are accessible by the search engine bots, I'm happy to say that they do still recommend that your pages 'degrade gracefully', ie that users without the latest whizzy browser can still get at your important content.