My thoughts, posted on a forum I frequent:

I'm a Christian (duh), and have never put truck in astrology. I do, however, acknowledge the importance of the four terrestrial elements, fire, earth, wind, and water to various belief systems. (For what it's worth, I'm earth aligned, and I just read that so is my Sign).

The zodiac is based on these elements, as well. Adding a thirteenth sign will throw this off and make it impossible to balance the elements. The Babylonians and Greeks who developed Western astrology weren't just watching the stars; they were mathematicians. Making discovery after discovery, from the number zero to the pythagorean theorem, these people used math to create an elegant and simple system to predict order in a chaotic world. Using a prime, and a particularly heinous prime (many Americans are triscadecophobic), defeats the whole purpose of astrology.

I'm sure you knew most or all of this already, but I just thought I'd show my support.