I write tests today to make sure it works tomorrow. As projects progress, inevitably no matter what kind of ninja coder you are, a requirement that is beyond what you could have imagined will pop up. You can either say, no we can't do that... loosing a competitive advantage. Or you can can code without fear. Because when you're done, you have a full suite of sanity tests waiting to make sure you didn't mess things up. Unit tests can make an average guy like myself appear to be that Ninja coder those job ads are always asking for, the guy with the oakley glasses.