The fallback isn't there because there's a public system call interface: the fallback is there because some of the kernel-side implementations of gettimeofday() (in particular, the Xen one) currently require the process to do a proper syscall.

This is separate from the fact that the gettimeofday() system call still exists too, which is a backwards-compatibility issue. The overwhelming majority of Linux applications do their system calls through libc too, so this doesn't affect them.