Designers should have the authority to lead decisions about how an interface should work and what it should look like, without a need to prove that what they say is right or wrong every time there is disagreement in the team. They are supposed to have the experience and knowledge to make the right choices.
Usability metrics, multi-variance testing and usability testing should all be adopted with great care. Continuous testing can be useful in many ways, but it should not replace informed design decisions.
Luca Benazzi, The tyranny of testing over design