What do you think the VDSO is used for? It also exports "functions potentially useful when called from the program".
The approach is a little different though; ntdll exports all of the NT API, and you need to go through it to reach the NT API in a somehow more stable way than using syscall numbers. OTOH, the VDSO exports only virtual syscalls that gain (or have gained in the past) from being performed in userspace, and even then corresponding syscalls still exist in the kernel, with both stable numbers and even a stable API.
