First of all, as others mentioned, TEDx events are independently organized. There are over 3000 of them in the world and obviously quality varies greatly. Getting a TEDx license is pretty trivial and there is no real oversight on quality. Yet, there are some great videos out there.
Second, nobody pretends TED is an academic conference. I see a TED talk as the blurb on the back cover of a book. The speaker's job is to pique your interest in a topic during that 18 minutes. Pique it enough that you'll go on and research the topic in greater detail. Nobody expects to be a master in anything after sitting in a chair for 18 minutes. But if you've never thought about a problem, 18 minutes may push you to do it. And it's true some talks are mostly inspirational, with little informative value - we usually put a couple in the lineup as a breather.
Third, TED is about cross-pollination of ideas. You hear an idea in neuroscience and it inspires you to do something in CS. Happens all the time. You will not act on 99% of the information you learn (be it in news, books, internet, HN) anyways, but it does expand your horizons.
Lastly, TED's biggest value is in developing countries. If you live in NYC or SF, there are dozens of conferences you can attend every week. So the marginal benefit of going to a TED event is little. However, TED as a brand is really well known in developing countries in Asia, Africa, and Europe (like mine), inhabited by few, if any, world class innovators. In those countries, people do find TED really inspirational and often the local TEDx events are one of the very few decent conferences you can attend.