Of course tau * i is what's really fundamental; however it's much more convenient to have the notation refer to a real constant. Firstly, one might be working in a context where complex numbers are not present, and to have to use them just to even refer to a real constant would be an annoyance. Secondly, if one defines a real constant tau, it is then easy to talk about the imaginary version i * tau; whereas one defining an imaginary constant, and having to divide by i or multiply by -i to get the real version would be somewhat annoying. Thirdly, if one went with the complex version, there'd be the whole "i or -i" problem due to the symmetry of the complex numbers -- OK, I guess this is not really an actual problem, but it would be slightly annoying, especially in context where dealing with complex numbers at all isn't really necessary. Whereas defining tau to be a positive real number gets rid of that problem.