A JavaScript file failing to load will bork most pages. A CSS file failing to load or a key image will cause most people to quit. And an Ajax request failing in a single-page app will render it useless.

But, my point of view is from actual Selenium users. This is framed by providing support on the IRC channel, on the mailing lists, triaging the issue tracker, and by interacting with people at SeleniumConf and the local Boston meetup. It's not some fringe use case and I'm not arguing the point for the sake of arguing it. The original supposition that it's an edge case is not accurate. And sure, the web breaks. That's why people using Selenium would like a way to catch that. And that's a big part of why the BrowserMob Proxy project exists.