The Crescent Bay prototype used the drivers from porta-pros, which are pretty good. The point of included headphones:
A) Based on Twitch and YouTube a lot of people didn't use headphones, losing most benefits of headtracked and spatialized sound. Integrating them makes them much more likely to be used.
B) Standardized frequency response and built-in DAC/amp means their HRTF (head related transfer function) spatialization tech can be tuned precisely, without worrying about the EQ of thousands of different variations. Though people are still free to use their own and lose the fine tuning.