The gain is pedagogical: giving kids a good intuition about angles is so much easier when the constant you're working with represents an entire turn around the circle (360°) rather than a half-turn of 180°. The advantage of using tau instead of pi is much smaller in other situations, but when it comes to measuring angles in radians, it's huge. And kids who have a better understanding of angles and trigonometry are just a little bit more likely to become good engineers. So persuading math teachers that there's a better way to teach trig is an investment in the future whose potential payoff is 20-30 years (or more) down the road.