I've read about NASA Glenn's Next's and JPL's DAWN ion engines using xenon (or cheaper argon and krypton) to go on trips to asteroids, bring a small one back to lunar orbit, bring back mined minerals, and Ad Astra's one to deflect asteroids.
These ion engines all use neutral gases, bringing several tons of fuel with them. If the craft could use gases extracted from the asteroid itself, there would be plenty for deflecting the asteroid or bringing it nearer to earth for mining, and it wouldn't have to launch with as much fuel.
While an ion thruster that uses in-situ resources sounds at first glance like a good idea, there's probably a reason I haven't seen articles about them.
Would such an ISRU ion thruster be infeasible or otherwise inferior to other ways for moving an asteroid?