That could be ideal.. The Asteroids could be a good Colony Ships.
A very interesting idea.
[Note, this post is pure conjecture, as I know nothing about the orbits of asteroids. My apologies to any astronomers injured by reading my post.]
They already have a whole bunch of mass that could be used as structure and (I assume) radiation shielding, and that mass already has momentum. That's a whole lot of mass/fuel we don't have to battle gravity for, or find in space.
I doubt they would be of much use for taking us where we want to go, but they could be wonderful tools for learning about long term, high population, space travel. Probably want to throw in a bunch of sensors to map part of the system for "free" at the same time.
If we got really lucky, maybe we could exploit their momentum and gravity to use them as tug boats for bulk shipping supplies to in system colonies.