An MPP implementation of MPI could use that information to match processes to processors in a way that optimizes communication paths. MPI is totally portable. Recompile and run on any implementation. With virtual topologies and efficient buffer management, for example, an application moving from a cluster to an MPP could even expect good performance. MPI is formally specified.
Implementations have to live up to a published document of precise semantics.
MPI is a standard. Its features and behaviour were arrived at by consensus in an open forum. It can change only by the same process.