From ONJava.com: Top 12 Reasons to Write Unit Tests
- Tests Reduce Bugs in New Features
- Tests Reduce Bugs in Existing Features
- Tests Are Good Documentation
- Tests Reduce the Cost of Change
- Tests Improve Design
- Tests Allow Refactoring
- Tests Constrain Features
- Tests Defend Against Other Programmers
- Testing Is Fun
- Testing Forces You to Slow Down and Think
- Testing Makes Development Faster
- Tests Reduce Fear