[toast0]

They still try not to break things, because you might be running a new kernel with old userland (this is part of the typical upgrade process), or you may have 3rd party programs that were compiled some time ago. I'm only familiar with FreeBSD; statically linked programs are usually good because old syscalls are typically maintained for a long time, dynamically linked programs will tend to be ok if you install the compat libraries.

There's been errors and exceptions of course.

I think the real benefit is they don't have to worry about people trying to run new userland with old kernels; that's explicitly not supported and stuff in base usually doesn't worry too much about it. So if netstat needs a new kernel interface to be faster, the netstat binary in the new release may not work with old kernels, c'est la vie.