The inefficiency is comparable to those who advocate using antimatter as the ultimate fuel, somehow forgetting that it needs to be manufactured somehow at great

The inefficiency is comparable to those who advocate using antimatter as the ultimate fuel, somehow forgetting that it needs to be manufactured somehow at great energetic expense. If one has a Dyson sphere’s worth of energy to power one’s antimatter-production plant for one’s ultrarelitivistic fleet, that’s great; but if you’re thinking of massive conventional fusion reactors as an intermediary might as well just take those (and their large amounts of light fuels) along for the ride and cut out the intermediary product.