Flatlander: The parable of the managers explains why thou must go forth and write unit tests.
The way of darkness: "And the newest servants, like he who had gone before, did again trample on the delicate codebase, but left footprints so light that they did not come to light until the system had been in production for a week, after which it crashed like a Volvo upon the motorway, and stayed down for two days;"
The way of light: "And when he trampled on the codebase, the tests of unit did send a red bar to warn him away from sin; and he laid on hands and did heal the problem even before he performed the check of in."
Unit test and documentation have saved my sanity on more than one occasion. This article is long, but well worth the read.