To be honest, when you factor in extra insurance, license and maintenance, having a high mpg commuter car only starts saving me money when gas gets to about $3 per gallon, but it does have lower emissions than the truck, and I can live with that.
Convince is part of it too. We picked up our "go cart" (00' Toyota Echo) for a few grand a year or so back (gas was $4.30 then and it really made sense but we still like having it) and its easy to zip that car around traffic, etc. compared to a truck. We keep work out gear in the car, etc. so it sees the daily wear and tear while the truck rests and ideally stays primed to roll for fun on the drop of a dime. In our case with 60+ miles a day in driving in larger metropolitan area commuting traffic I'd prefer this option. If my commute was only 10mi a day across town with low traffic I'd have to reconsider.