I think it is fun to hypothesize, to say "what if". Trying to solve the problems of a colony on Mars is a fun brainstorming exercise. Because we are working on brainpower, for fun, funding isn't an issue. And many cool discoveries and inventions have come from "what if".
The atmosphere on Mars is an interesting thing because it relates back to the basic question of "why does Earth have an atmosphere?". The atmosphere on Earth constantly drifts off into space, but not as fast as Mars because there is more gravity. And the atmosphere of Earth is constantly replenished by a complex cycle of subduction zones, volcanoes, and life itself. Mars wasn't dense enough, and didn't have the right materials to begin plate tectonics.
As a result, life may or may not, (evidence is growing but certainly not absolute yet) have formed in the beginning but did not get far before the planet began to run out of gasses as it cooled. (It even took Earth quite a while to get from single cell to multicelluar organisms.) CO2 is left because it is common, and weighs more than O2.
