I would guess yes. I mean, if you don't pay the carrier, it will just sit dormant.
But can you even buy the Watch without a plan. You couldn't do so with iPhones until Apple started selling them unlocked.
Its not like I can call from both at the same time.
I believe the concept is to use one when you're without the other. Business being what it is, carriers won't let us just pay for time when we need it. Instead the contrive plans to take our money.
I don't know if two cellphones with the same ESN and phone number could make calls simultaneously and assume they can't. And it seems trivial for the carrier to block the ability to use an S3 Watch and phone at the same time. So why even charge us at all. Take any data used from our phone plan.
If we BYOP we still have to pay a monthly 'access fee' on top of our plan fee. Why is that? Rhetorical. I know why.
So pay or don't.
My understanding is that cellular will be like GPS:
I have no knowledge or understand of how the S3's cellular will work. But since all previous Watches will allow you to make or take a call it surely seems possible to manage LTE like GPS as you suggest. I've taken a few calls on my Watch but except for some rare instances of very short calls, just to allow me to answer the call while digging out or getting to my phone.
I need to reduce my plan anyway. Adding the Watch to my collection would mean a) the potential savings of a new plan would be halved 2) it would be expected that I wouldn't have my phone with me on occasion to get calls, texts, and notifications (I generally have it with me) and c) that would be my only Watch because if I was wearing my S2 and forgot my phone, the S3 sitting at home would be wasted. Oh! Wear the S3 when I know I'm going to forget my phone!
Maybe a trip to att.com is in order. Is the $10 just another 'access fee' and the data and minutes come from my phone's account (which makes sense) or does the S3 have data/minutes restrictions from the phone's account.
Regarding battery life, I'm not concerned. Any calls would be very few and very short, knowing that the meter is running faster than normal.