I always thought the rationale was that if statically linked binaries are on different partition they can be used to recover the system from a failure.
Edit: files in /bin are also statically linked, and I am unsure about what I wrote above but vaguely recall something like that
> /bin/ User utilities fundamental to both single and multi-user environments. These programs are statically compiled and therefore do not depend on any system libraries to run.
> /sbin/ System programs and administration utilities fundamental to both single and multi-user environments. These programs are statically compiled and therefore do not depend on any system libraries to run.