How could you use this for manned flight? Assuming a (best-case) linear acceleration, accelerating to 25000 mph over 1 km would be 12736 g.
The astronauts would come on conventional rockets. But then they would be able to refuel from the orbital depots for the later stages of their flight instead of having to carry all fuel with them. Thus, you make the mission cheaper.
You still need really stable rocket fuel in order to handle those accelerations without exploding.