Only if it's accurate, which is apparently a problem with such tech right now. FDA shockingly allows 20% error in regular glucose meters.
The savings from not having to buy strips would more than pay for the watch.
The app itself would be useless if in fact you only enter "daily or weekly glucose levels". An insulin-taking diabetic is checking glucose levels 5-7x a day. I don't see a tab for medications either. If you're taking insulin you need to track that.
Doing an estimated HbA1c and graphs would be important too.
Alas, as we now know the watch does not have glucose monitoring, but the rumours are already starting that there will be more sensors in the next version. I am likely to pick up the current watch when it releases, but even if I do, I will buy the next one immediately if glucose monitoring is included.
I am looking forward to seeing the final version of the Healthbook app and I too hope it can track more than a daily entry. I have a Dexcom CGM as well as a fingerstick meter, and use both together combined with medication and insulin to control my sugar levels. I'm currently using a thirdparty app to record sugar levels, and it's not bad.
As you noticed, fingerstick meters can vary in accuracy. I tend to get higher readings from the meter I use (and it's a new meter) than the meter my doctor's office uses. There's also variation from site to site. If I measure from one forefinger, then the other, the readings often vary fairly widely. The Dexcom CGM also does tend to vary somewhat, partly because it measures via interstitial fluid and not blood but partly also because it measures at a different site than the conventional meter.
Devices like an Apple Watch (or similar from Samsung or other makers) will not fully replace FDA-approved devices and treatment from a doctor or nurse practitioner, but as the technology improves, we'll see them being used more often.
A Raman-spectroscopy-using watch that could noninvasively take sugar readings would be incredibly useful not only for known diabetics like me (thanks to a congenital condition, I had high risk and lost on the dice roll of life!) but for people who are diabetic or prediabetic and don't even know it.
If a future Apple Watch gives a reading of 200 mg/dL and the wearer calls their doctor and has a blood test done and starts treatment as a result, that's something that might have not happened had the doctor never had a reason to order the test. Remember, not all diabetics are overweight or feel sick. I had no idea I'd developed diabetes until I was told I had, and I have no idea how long my sugar levels were elevated before I went on medication.