> I can't think of non-entertainment contexts where AR is actually worth the effort over a screen interface.
This I agree with.
> AR via a phone/tablet seems to me to be just as good an interface as any headgear
This I cannot agree with. If you've used recent VR, you know it's the "real deal". We've caught up on the computing capabilities required for full color passthrough and even fairly lightweight headsets. Right now it's a matter of miniaturization, and bringing the cost down beyond the $10,000 price tag that current experiments are running.
If AR becomes a mainstream thing, people are going to want to ditch their phones the moment a good headset is available. If it's not (and I suspect it won't be; "AR" already exists on our phones and hasn't been popular since Pokemon GO) then people will remain apathetic.