A tractor beam may be great, but could throw the game balance off or not actually be used.
IMHO, asteroids should be left stationary. Presumably, a tractor beam would be mounted on a sufficiently large ship. The cost required to research such a ship and such a weapon/tech would be huge. This would be only mid to late game, and the only asteroids available for mining would be far away from a home base or any refinaries. Defending far off resources is an integral part of the game. It would have to take a long time to transport the asteroid. All in all, it would be a very expensive operation transporting just one asteroid, and for what kind of reward?
If one would want to transport asteroids, I'd suggest a different ship (possibly AI controlled) that attaches to the asteroid and has massiv engines.
Then, there is the idea of stealing other ships or miners or cons. Again, why would someone with a cap or other sufficiently large ship and late in the game waste their time stealing a single scout or miner?
I'd suggest a new ship for Belters that allows one to capture ships, miners or even pods, in the same style as SPECTRE's Bird one. This could be a cheap, but weak ship. The belters are the recyclers of the galaxy, so I'd expect them to have created something like this. To balance, the other factions could have weapons that could disable a ship for a period of time, slows it down or force an eject.