Sometimes I fear "You need to test" has become just a different way to say "I don't know."
"You need to test" can also be a cop out. You can learn a lot about what works just by looking and modeling your business after other successful marketers/companies. Or by doing a statistical analysis of what successful businesses have in common.
I'm all for testing and probably do more than most people (including for every client I write copy for). So I respect the need to test.
But that said, I think stating "You need to test" can actually be the WRONG answer when there's plenty of evidence already pointing in one direction.
Testing may always be the final answer, but modeling success should often be the first step.