I thoroughly believe that testing actually helps you think about the problem. That being said, I don't think you need 100% coverage either. Automated test suites give you a "warm fuzzy feeling" that given certain inputs, you get expected outputs. It also helps you see if there are any edge cases that you haven't thought about. To counter his point, I've encountered many people who haven't wanted to test because they're too proud of their code, too lazy to learn how to do proper testing, or can't do it "because their architecture isn't set up for it" (aka, it's bad architecture).
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