It matters for Apple, for developers? not so much. Millions of users and not a single third party "killer" app.
Being able to check messages, calls, Uber, flights, etc. right on my wrist are all killer applications for me, along with the fitness features of course.
I suspect the upcoming walkie-talkie will be fun too (something that should’ve been there from the start.)
> Which developer wants to miss out on serving potentially millions of users?
You are forgetting that most developers already have millions of potential users on the iPhone with a market at least 30x bigger than the Watch's, and iOS has none of the limitations the Watch has.
Right now a Watch app requires an iPhone app, is hard to monetize, it cannot be sold on its own, it's slow and frustrate your users, etc. The only upsides are for Apple. It might sell really well because the core functionality provides enough value, but developers need to see a clear return for their efforts and right now is not there.
Right now there is no reason for Youtube to make a watch app, and no reason for you to pick Vimeo over Youtube because it has a watch app.
I've seen users requesting a Watch version in reviews for iOS apps. Even I have chosen some apps in the past over others specifically because they offered a Watch app, like navigation apps and flight checkers etc.
As a user though, I must say that if "monetizing" and "selling on its own" are more important concerns for you than make your product/service a better experience for ME, then I probably wouldn't want to use any of your apps.