I think what SteveS is talking about is
the problem with catching up to the cycler if you want to catch a return trip home.
A downside of the gravity-assisted Cycler concept, however, is that the vehicle flies by Mars at quite a high speed, up to 27,000 mph. This velocity is not a showstopper on the outbound leg, where the CEV taxi craft would aerobrake, relying on the friction of the Martian atmosphere to slow down without using any propellant. But departing Mars for the leg back to Earth, the craft would need a large amount of propellant to catch up with the speeding Cycler.
Buzz envisions a semicycler to solve this particular problem.
This problem may or may not be an issue for the asteroid based cycler. Buzz was able to plot a 26 month cycler orbit based on gravity assists from earth and mars. One thing is certain, moving an asteroid close enough to earth for a gravity assist is out of the question. We would use the asteroid's current orbit which in all likelihood would be much longer than 26 months.
Definitely sci fi based on our current space technology but hard sci fi.