Bringing propellent rich asteroids to the earth moon neighborhood is an interesting idea. There are a number of entities pursuing this.
However most proposals suggest solar powered ion rocket engines for asteroid retrieval rather than a vehicle driven by a series of nuclear blasts against a pusher plate.
The U. S. is thinking of developing a solar powered asteroid retrieval vehicle. It would be part of the Asteroid Redirect Mission (A.R.M.). The A.R.M. vehicle is expected to be modeled after the vehicle described in the Keck Report.
However A.R.M. is not being sold as an enabler for using asteroid resources. The mission is promoted as an intermediate step to Mars (a foolish sales pitch, in my opinion).
Critics will say politicians embrace A.R.M. because it's doable by SLS and they want to justify the SLS pork barrel program that keeps folks employed in certain congressional districts. They further contend that SLS missions to an asteroid in lunar orbit do little to get us closer to Mars. I believe the critics are correct. But what the politicians as well the critics miss is the potential usefulness of the A.R.M. SEP vehicle.
Planetary Resources has made a nice video giving a layman's explanation of Tsiolkovsky's rocket equation and delta V budgets:
Aseroids aren't the only possible source of extra-terrestrial propellent. Paul Spudis is an advocate of using lunar water as a propellent source. There may be large deposits of ice at the lunar poles. Another advocate of lunar propellent is Bill Stone of Shackleton Energy Company.
Extraterrestrial propellant, whether asteroidal or lunar in origin, would require substantial mining and processing infra-structure. This is an extremely expensive barrier. It may be a large enough hurdle that we will never make use of extraterrestrial water.
There are those who argue captured rocks offer no military advantage. It's true kinetic kill weapons would be horrendously expensive. Nuclear bombs could wipe out a city for a small fraction of the cost. But those using this very obvious argument are stuck in 1970's science fiction. There are other possible military uses for space resources than throwing rocks.
What extraterrestrial propellent might offer is routine access to orbital assets in cislunar space. This includes surveillance satellites, GPS, communication satellites and weather sats. Easy access to these would indeed be of incalculable military value.
As suggested I will elaborate on what I mean by routine access. Presently a ship from earth's surface to LEO has a 9 km/s delta V budget. Returning to earth's surface entails enduring the extreme conditions of an 8 km/s re-entry through earth's atmosphere. So far economical reusable vehicles haven't been realized.
In contrast vehicles between earth orbits would have 3 to 4 km/s delta V budgets. Most wouldn't have to endure the high temperatures and dynamic pressures of re-entry. Those ships with LEO destinations might use aerobraking for circularizing at LEO but the aerobraking would be much less extreme than an 8 km/s re-entry.
With the present paradigm the routine is launch, use and discard. With cheaper transportation between orbits it would be possible to upgrade and maintain existing assets rather than letting them die and add to the population of orbital debris. It would also be possible to salvage dead sats.