I agree with r0j0hound, about the fact that it is not because it is easy that it shouldn t be included:
First: readeability, just reading Sign() is explicit enough, contrary to the other expressions we can set up for it
2nd: I had more trouble doing it that doing a lerp with expressions, because of the 0! And it is easy to forgot for a non mathematician guru than 0/0 does not exist (lerp does not need any conditionnal, it is a simple calculation, while Sign is not a calculation, but an evaluation at the end)