py-h5py wraps the HDF5 library. It contains these lines:

    depends_on('hdf5@1.8.4:+mpi', when='+mpi')
    depends_on('hdf5@1.8.4:~mpi', when='~mpi')
    depends_on('mpi', when='+mpi')

The first two of these should probably read instead

    depends_on('hdf5@1.8.4:')
    depends_on('hdf5+mpi', when='+mpi')

This would allow using a plain h5py (which is h5py~mpi) with a hdf5+mpi. I don't think there is a reason to disallow this.