glibc-2.8 from 2009 didn't use any vector optimizations, even though all AMD64 CPUs support SSE2, and those came out 6 years ago, so the excuse of maintaining compatibility with all CPUs of that class doesn't hold. For example not all i686 CPUs support MMX, so the non MMX equipped Pentium Pro could be the reason for not using MMX. It probably got to the point where the threat of outside patches to glibc motivated them to add support.