Pi is not globally reserved. It is commonly used in, for example, statistics to refer to multinomial probabilities, and wikipedia tells me it's also the name of the prime-counting function and parallax.
I should note that this was annoying to me when dealing with IRT, in which some models have 2pi in the normalizing constants. Not to say tau isn't used in statistics already as well.
There just aren't enough greek letters. I have wondered about what the symbol for tau should be, and I keep thinking of a circle including a radius line that extends slightly outside the circle, but tau is much easier to write, especially when using ascii.
I should note that this was annoying to me when dealing with IRT, in which some models have 2pi in the normalizing constants. Not to say tau isn't used in statistics already as well.
There just aren't enough greek letters. I have wondered about what the symbol for tau should be, and I keep thinking of a circle including a radius line that extends slightly outside the circle, but tau is much easier to write, especially when using ascii.