Comment by samus

Glibc and the situation on Linux is not involved at all here; we're talking about OpenBSD that takes invoking system calls via libc very seriously.

The libc can enforce some minimal sanity checks on the syscall arguments. For example the number of arguments and some early validation to avoid entering the kernel if failure can be foreseen. But the main utility is to make it more difficult to use Return Oriented Programming to build exploits.