While I'm a habitual nitrox tester, I actually didn't check my nitrox mix on my last dives in Cozumel. Not my cup of tea, but there wasn't an analyzer on the boat and I didn't bring my own. Still, faced with the choice of sitting out the dives versus risking my life based on someone else's analysis, I'll choose to risk my life almost every time. I'm going to order a new O2 sensor for my old analyzer before my April trip and bring it with me this time.
The difference is that, especially with partial-pressure blending, it's easy to get the mix wrong. When multiple tanks are being filled/analyzed, it's easy to mislabel a tank. Heck, even when there aren't so many tanks being filled, it's easy to mislabel a tank. We picked up tanks from a south Florida dive shop once, right at closing time so we didn't analyze them at the shop. Fortunately I had brought my analyzer with me that time and when I checked the tanks the next day, we noticed that the 35% and the 28% mixes had their labels switched. Had I not analyzed the tank, poor J would have exceeded a ppO2 of 1.8 when we hit the sand at 140' diving the Spiegel Grove. Given the time we actually spent at depth, I doubt that would have killed her. However, when oxygen toxicity hits, it's not a matter of getting a headache or nausea, it can cause a seizure at depth which ultimately leads to drowning or embolism unless a rescue is successful.
When filling tanks, it's the object of decent fill stations to minimize CO and other contaminants in the tank by using filters, oil-free compressors, strategically located air intakes, etc. On the other hand, when partial-pressure blending, the fill station is deliberately introducing the "poisonous" oxygen into the tank. With CO, the idea is to analyze to see if there is accidentally a level of CO that is too high for comfort. With nitrox, you already know there will be a significant amount of oxygen in the tank, the question is how much. Even in Cozumel, the vast majority of tanks do not have CO levels high enough to concern divers no matter how deep they go. On the other hand, 100% of nitrox tanks filled on the island will have a dangerous level of oxygen if the diver exceeds a certain depth. That certain depth can only be determined by analysis of the gas, so analysis is absolutely necessary with inaccurate partial-pressure blending and still a very good idea even diving nitrox pumped from banked gas or a membrane system. The need to measure is absolute and it's usually done at the filling station. The reason why a diver re-checks the mix is because he doesn't want to trust the busy filling station to get it right all the time. (Besides calculating MOD from the mix, it also determines the dive computer setting. If the mix is actually lower than the computer setting, the diver runs the further risk of unknowing incurring a deco obligation.)
So while CO and nitrox are both gases, they're apples and oranges as far as measuring fruit goes.