I assume you're asking about my example and to clarify that should have been me owning a Prius, as I can't speak for everyone in general. I only need a vehicle to go from point A to point B. My truck costs more in it's monthly payment, gas, insurance and general maintenance than a Prius would, yet the Prius meets my needs 95% of the time. The savings would be more than enough to cover me renting a truck on the occasion I actually need something bigger. Essentially, me owning a truck instead of a small car is wasteful and I only own one because I wanted one not because I needed one.
Ok, I thought you were saying Prius for the low gas costs, but if you were a minimal driver, gas costs are a low component of your costs comparative to vehicle price, so if you had a cheap truck that would be more economical.