Almost everyone I've heard from -- in person and online -- that has had a Pebble or similar smartwatch has said that getting notifications on it reduced their gadget-focussed attention compared to having only a smartphone, and cited that as the primary benefit.
So, while I think it could be, for some people, a new distraction, I don't think that's generally the case, and is, in fact, the opposite of what it normally is and what is motivating people to buy it.
I don't see how wearing a watch that gathers biometrics would be unhealthy. A small bracelet without any display would be fine with me. At the moment, I don't need this - I'm in my prime. But once I get older, I might benefit from having a small buzz on my wrist alerting me that I'm working too hard mowing the lawn, or whatever.
I'm also against a model such as the one you describe, like I have made clear in my post.
The pebble demands your attention. That is all. It does nothing that something else doesn't do already, costs a chunk of money, needs constant feeding and attention. At least a smartphone was a consolidation of communication and entertainment devices. This is a portable distraction. A poor one at that.
Do you think it's healthy to constantly stare at your heat rate, glucose level and blood pressure and micromanage every parameter of your life?
Also in a decade, you will pay a subscription fee for that data from the central provider that your watch talks to. That model is established already plus it will probably be linked to your health insurance profile as well by then. You'll wear one to keep your premium down...