If the designer displays advantage, she lays her design open to unnecessary criticism.
If the programmer displays advantage, she lays her code open to unnecessary criticism.
When you have an advantage, use it for the benefit of the customer: ExtremeHarmony requires (or appears to) that the shape of the design, and the shape of the code, becomes known almost by removing what is not required. Advantage for whom?
Clearly a harmonious development environment is desirable, if only for the lower stress levels it brings. In an aggressive environment a display of advantage will start your opponents thinking how they can nullify your advantage. And the only benefit you've had is an illusory feeling of ImRightYouAreWrong? for a short space of time.