Every process must be able to make system calls. This is after all the mechanism by which the system libraries will interface with the kernel.
The problem is the system's developers don't want us bypassing those libraries. We can do it but things can and probably will break in the future when they change things. It's not supported.
Personal experience.
> Do they guarantee the system call numbers will remain stable though?
No. Doesn't mean you can't make system calls from outside the libc though.