Actually I thing using asteroids this way will become fairly common.
It seems far fetched on it's face but after a little thought it starts to sound more feasable than building large spacecraft on earth and launching them or building them in orbit.
Someone (perhaps Asimov, but he probably wasn't the origin of the idea) suggested using comets to explore the outer solar system. The idea was to stick a probe on the comet and hitchhike out. There are too many problems with the idea of course, not least, that hanging on for the ride may be too much, and the damage to the instruments and communications may be fatal.