The export and import of the RFC's is good only if you want to retain all the RFC's.
If you want to retain all the RFC's in the way it was in the old system then you may export them.
The draw back is that you will need to modify the RFC's once they are imported with the correct user name and password as once the new system is built then old user master records won't be available.
If there is only few RFC's then it is worth recreating them after the new system built.
Regards
RB