Re: Thanks for the explanation
"It would be very annoying if your watch 'rang' every call you got if your phone was next to you."
It does that anyway. And no, it isn't annoying, once you decide to treat the Watch as an extension of the phone that sits on your wrist and also tells you the time rather than a separate device in its own right.(1)
That is, it's no more annoying than hearing the land-line phone in the bedroom ringing at the same time as the one in the living room because they are on the same line.
The current (pre-iOS11) idea is that you can use the Watch to see who is calling (it shows that information on its display) and decide to answer or to reject or whatever and then if you answer, you take the call on your hands-free that you're already using. The hands-free sets I've used have all had no (obvious) way to *reject* a call, so without a wrist display you're left listening to it ringing and wondering who it is (because the caller doesn't have a per-contact ring-tone attached for whatever reason and the phone is stuck in your pocket).
(1) A Series 2 has enough extra hardware (notably a GPS receiver) to have a limited independence(2), but it's still essentially an extension of the phone.
(2) You can go open-air swimming with it without having to find a way to carry your phone in the water, and you can still get a GPS track from the swim time. As I said, *limited* independence.