The main use case for Newlib is embedded systems, and Newlib supports more architectures than FreeBSD does. Newlib also includes a few assembly routines for functions like memcpy and memset.
Sure, my point is, the question isn't use newlib or write your own, you can probably pick pieces out of it, rather than using the whole thing, by avoiding their build system and just using their source. I just don't know if the code (or license) is structured for that.
