Some projects make it possible for you to generate verified code from the specification.
There is usefulness to these tools even if the implementation is a separate beast. At a minimum, they allow you to test drive your ideas in particularly thorny areas such as concurrency and security. Forcing you to think hard about a secure kernel or a library of data structures is a good thing in and of itself. You only have to give such core libraries this kind of anal attention, not to the layers built atop.
There is usefulness to these tools even if the implementation is a separate beast. At a minimum, they allow you to test drive your ideas in particularly thorny areas such as concurrency and security. Forcing you to think hard about a secure kernel or a library of data structures is a good thing in and of itself. You only have to give such core libraries this kind of anal attention, not to the layers built atop.