In many documented tragic residential CO incidents the source of the CO has been a malfunctioning furnace / flue system - not a typical fire condition. The CO detector is the only detector able to detect this deadly gas condition in these instances. CO gas is odorless, colorless and deadly. CO gas is typically discharges hot causing it rise to ceilings, even though it is of almost identical density to air, the reason why ceiling detectors are my first choice.
I advocate using separate CO detectors when possible to optimize their effectiveness by positioning them in locations CO gas is likely to originate from, such as along the path of the chimney line especially in older homes (often passing through a bedroom), in bedroom hallways, at tops of stairwells, and reasonably near the possible CO sources.
Something is better than nothing; Combination smoke / CO detectors are better than no CO detectors!
Another important consideration is the presence of natural gas (methane) or propane gas, especially residential buildings. Often, unfortunately, overlooked by most security alarm sales persons, the vast majority of “Burg. /Fire” home systems do not have these types of gas detectors when needed. Just as deadly, these gases can cause violent explosions in the “right” concentrations. As in the case with CO detectors, these gases can have different sources, and “weights” so the detectors must be properly located. This is another reason why those "free" or low cost alarm systems can be the worst possible choice. For more information visit the National Fire Protection Association NFPA web site. Keep Safe! Felix