This has always fascinated me. We can actually build giant spaceships (space stations) inside asteroids and use them for interstellar travel. All the resources we need, including ice would be at our disposal. Like traveling with your own mini-planet.
Dirk Schulze had taken that as a subject into his sci-fiction book - Alien Encounter.
From an energy perspective, it's likely better to build your own station (from an asteroid - or alternative, remove all the mass from the asteroid that you don't need) - that way, you're only accelerating exactly the mass that you need, not wasting any energy accelerating the rest of an asteroid.
If it's a space station, then accelerating it is less of a concern than keeping it livable, and a huge amount of matter surrounding you goes a long way to protect you from things that would otherwise kill you.
Now for a space ship, I agree. Though even then you might benefit from that extra matter in the same way.
I think he is talking about chilling on the asteroid while it follows its known trajectory undisturbed, simply because they are packed with useful resources we can gobble up while waiting.
Asteroid are per definition not interstellar though (until Oumuamua?).
An interesting approach would be to send an automated ship builder robot fleet to dismantle an asteroid and board the fully furnished ship when it makes another close approach, and then boosting it to its final destination.
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.
Also mentionned in Legend of Galactic Heroes by Yoshiki Tanaka. The founder of the Free Planet Alliances escape from its prison world by using ice asteroids as vessels for interstellar travel.
Dirk Schulze had taken that as a subject into his sci-fiction book - Alien Encounter.