The onward connections are nice to have but realistically are only there to top up the passenger numbers on the main route. I would guess that 90%+ of the time the direct flight from Belfast would win out but if the flights are going from MAN it's as well to offer them as you'll always pick up a few passengers here and there.
Now if they were offering fully integrated connectivity to the Air France network via MAN/CDG that could be a winner, but as of yet they are not.