It may be useful as fuel for a mars vehicle or for rockets (electric vehicles will take you not very far and it's hard to strap the nuclear reactor to it). CO is easier to store than hydrogen. Of course, if you have a steady supply of ice you can make methane/methanol which is better for that purpose. But to extract ice you will need heavy mining machinery, earth-moving etc...
CO + O2 fuel would make more sense on Venus, where water would be more scarce (water is present in thin clouds of sulphuric acid in about 25% concentration). The surface, however, is another business.
Also I'm not sure how useful CO fuel would be if you had electrical power already. Maybe splitting CO2 into CO and O2 only to later recombine could make sense as power storage? Maybe if you are using solar power and regular batteries aren't feasible for some reason, or maybe in a vehicle?
There might be uses for burning CO on Mars, but not as the byproduct of CO2 splitting for breathable oxygen.
