This is another case of overgeneralizing based on limited experience. Automated unit testing may well be of limited value for building the applications that run a video sharing web site (where failures are no big deal). But now try building a complex library used as a core component of several packaged applications. The library developer doesn't really have a "program" to put through user testing. The only solution to keeping the library stable while refactoring and adding new features is to build a comprehensive suite of unit tests that exercise the library's public API.