If we possessed the technology to terraform Mars at some point in the future, what are the reasons to do it or not do it?

I tried to transform the yes-no question “If we possessed the technology to terraform Mars at some point in the future, should we?” into the Fluther format and wonder whether it really matters.

The terraforming of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, or ecology to be similar to those of Earth in order to make it habitable by humans. The term is sometimes used broadly as a synonym for planetary engineering in general. Mars is considered by many to be the most likely candidate for terraformation. Much study has gone into the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. However, a multitude of obstacles stand between the present and an active terraforming effort on Mars or any other world. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics and methodology of altering the environment of an extraterrestrial world.

There’s one idea about crashing a comet (or a series of comets) onto Mars to raise the temperatures. The approach was described by Marshall T. Savage in his book “Colonizing the Galaxy in Eight Easy Steps”.