Apple watch app without functionality
I would say you are over thinking the watch app.

Watch apps on Watch OS 2 and iOS 9 are extensions. They are the antithesis of a functioning app and simply a little detail or a little tidbit.

Furthermore, Apple approves apps that do absolutely nothing without a paid account. Netflix, Casper Focus, Splunk, all the newsstand subscriptions where you need to pay for content before reading an issue.

Now - Apple's review guidelines let them have great latitude to deny apps for no reason at all - let alone 2.12 "not very useful" which itself is highly subjective.

What you are concerned about also concerns many other app developers and it sure would be nice if there were a way to pay for "Pre-Approval of Innovative Features" so you could invest maybe 5k of effort to prototype and then pay for a review by Apple before investing the actual amount of development work in your app. See this story for the details of how pre-approval and the developers surveyed are in similar situation wondering what might pass review:


https://www.macstories.net/stories/developers-apples-app-review-needs-big-improvements/