The x86_64 ABI mandates a 16 byte aligned stack. I think Apple's just doing it in their 32-bit ABI as well to keep it consistent.

Regardless of the rationale, it really isn't a big deal at all. He'll need to jump through the stack alignment hoops anyway when he gets to finishing his 64-bit compiler, and so what if he has to do the same with the 32-bit compiler?