Stack protector warnings?

[nia <nia%NetBSD.org@localhost>]

To build many network-related and otherwise security-relevant
parts of src we use:

COPTS+= -fstack-protector -Wstack-protector

For pkgsrc, we use -fstack-protector-strong. Many others are
also using this, and it seems like it's more useful (it
does something that seems "obvious" to me, protecting functions
with local array definitions).

However -fstack-protector-strong seems incompatible with
-Wstack-protector -Werror (enable this and watch as
src fails to build).

Is there any value to -Wstack-protector? What useful hints
does it provide to the programmer? Isn't applying the stack
protector entirely the toolchain's responsibility?