Carrying a knife in the Cozumel Marine Park is a violation of the rules.I may, or may not sit out the dive at 5ppm, but knowing the CO level in your tanks is a good thing to know before diving imho.
The same could be said for a lot of things. I have carried a knife on most my dives and have never never used it, but I still carry it even though statistically, I probably won't need it.
If they were smart, they'd ban CO analyzers too.