Ketones are only going to be present if you are exceptionally bad (very high carb diet) or exceptionally good (very low carb diet) at managing your blood sugar. It's not going to be terribly useful for your average diabetic who has fairly good control over their blood sugars.
This is not correct. When someone with type 1 has an illness they can develop ketones while their glucose appears stable within range. This is can lead to death. About once a year, when my son is sick, we need to monitor his ketones with finger pokes. When he has a stomach bug and can't eat, we have had to go to the hospital for a drip to allow us to give insulin and lower his ketone level. Having ketones on the CGM…
I presume you are referring to https://www.ncbi.nlm.nih.gov/books/NBK554570/