QUnit is especially useful for regression testing: Whenever a bug is reported, write a test that asserts the existence of that particular bug. Then fix it and commit both. Every time you work on the code again, run the tests. If the bug comes up again - a regression - you'll spot it immediately and know how to fix it, because you know what code you just changed.
Having good unit test coverage makes safe refactoring easy and cheap. You can run the tests after each small refactoring step and always know what change broke something.
If you are interested in helping developing QUnit, you are in the right place. For related discussions, visit the QUnit and Testing forum.
Planning for a qunitjs.com site and other testing tools related work now happens on the jQuery Testing Team planning wiki.