1) First of all if you wish to save the environment, then you should not buy gas powered motor vehicle.
2) Little commute is not as Eco-friendly as you think it is because you make more cold starts. During the cold starts unburned gas goes directly to motor oil; therefore, causing the oil to lose its properties faster. According to the factory recommendation you still must change oil at least every 6 months that means 70 km/week x 26 weeks = 1820 km. Compare it to 10,000 km in normal use.
3) If you think by burning less fuel you are not harming environment, then you are not exactly right. Let me compare the honda civic from the 80's and the current one. When that old civic was new it had better mpg than current one. The current one drives better, much more safer, more comfortable, and has better features and we pay for having a less efficient car.
However, the current Civic is more Eco-friendly than the old one because of it has better neutralizing exhaust gases. For another example, if you decide to commit suicide with the latest model of a car with its engine on, parked in closed garage, you'll die not of the toxic exhaust gases, you'll die due to lack of the oxygen. And you'll probably not die because the engine will not work without enough oxygen anyway. That means your new Mitsubishi Mirage is much more Eco-friendly than your old Geo Metro, even if the Mirage burns more gas.
5) More people buy the Mitsubishi Mirage due to its fuel efficiency. Compare it to the Chevy Suburban, the Mirage uses lot less fuel, that means it spends lot less money. Making much less harm to the environment by driving the Mirage is a huge bonus.