i can't tell you how many failed trig tests happen as a direct result of a non-intuitive unit circle defined in terms of Pi [1].

Pi is fundamentally wrong. A circle is defined by a point and a radius. Diameter is just something that has always been easier to measure, so now we're stuck with it :( The fact that Tau makes some formulas less "elegant" is irrelevant.

[1] http://www.shelovesmath.com/wp-content/uploads/2012/11/Unit-...