It's incredibly useful, if you need functionality that's not yet supported in the range of browsers that people use.
I'm pretty sure I can't handle MIDI and audio input, do pitch detection, show animations synched with generated audio, etc. in JavaScript on IE6 (and only parts of that are possible in any browser) -- but I can do it with a Java applet.
Or, I could; my site is obviously starting to suffer from the reliance on Java, and every few days the news seems to get worse.
Not everyone can choose their browser -- either because they're still running Windows 98 and can't afford a new computer, or (more likely) because they're part of a large organization that can't afford to upgrade due to huge past investments in aging tech.
I actually deal a lot more with IE6/7 in my main job (users in the NHS) than for the music theory site, but to my main point above -- there are plenty of audio features that are just now starting to be possible in Chrome (no other browser, yet); that's simply not useful in the context of real, normal-people users.
Tell that to my organization, which is still stuck in IE7 despite a paid-up ongoing IT services contract. They even managed to block out Google Chrome Frame somehow.
I'm pretty sure I can't handle MIDI and audio input, do pitch detection, show animations synched with generated audio, etc. in JavaScript on IE6 (and only parts of that are possible in any browser) -- but I can do it with a Java applet.
Or, I could; my site is obviously starting to suffer from the reliance on Java, and every few days the news seems to get worse.