Now for a space ship, I agree. Though even then you might benefit from that extra matter in the same way.
Asteroid are per definition not interstellar though (until Oumuamua?).
I'd love to see a scenario where we'd build robot terraforming fleets that'd land on an uninhabited planet, make it habitable, complete with spaceports, cities, factories, crops (all permanently tended for robots), build a couple extra fleets and launch to their next most promising targets.
And then, for some reason, humans lose interest in all that and someone else wakes up to find a universe full of 60's futuristic utopian planets nobody ever lived in.
Reminds me of the paperclips game [1]. Would be horrid to find those people wake up and find the universe transformed into nothing but paperclips. [1]