XS has two typical uses: one is writing interfaces to external libraries, that's also what FFI::Platypus does.
But then, XS is also a way to extend Perl in its own. It is another way to write modules, for instance, when performance matters or when access to the runtime internals is required to do something. I don't thing Platypus supports that.
I have zero experience using Platypus, but I doubt any library exposing a clean FFI would allow me to do the tricks I have done in XS sometimes... though, I would never say that disallowing those would be an unacceptable trade off!