Heidegger has a throwaway line about how "the long-range airship [the Highwind, I assume] is still being prepared".

As for the buggy - I guess you can simply bail out and run to Cosmo Canyon before it breaks down, though I don't remember what effect - if any - that has on the events and dialogue that follow.

:edit: And having the vehicle break down means that the rest of the party's forced to visit the Canyon and accompany Nanaki, rather than having him be all "ok g2g cya!" and running off home, leaving the others to continue on their way.