Obviously, the killer use case is that it would let you ditch the phone altogether. That takes care of your first objection since there wouldn't be an extra data plan.
Your second issue will remain, but there's a huge tradeoff: no extra thing to carry around in your pocket and pull out to fiddle with all the time. To compensate, we'll want to see voice control getting better and more comprehensive. You'll end up using larger-screened devices (that you don't carry around all the time) for things that need a larger UI.
Also, your third objection isn't a drawback but a technical hurdle to be overcome. It's the main reason we don't already have this. Let's assume that gradual technical improvements will alleviate this issue, sooner or later.
Personally, I think the mass switchover from phone to watch will be very quick once they have the cellular communication capabilities and a robust/reliable voice interface.