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Probably because if it's working right, you'll get a notification on the watch and that's easier than a glance. Problem is that it doesn't always work right.
 
The only passbook item that has successfully popped up at the appropriate time has been airline boarding passes.

Starbucks, Dunkin Donuts, etc cards do not pop up when at locations.
 
I agree. I wish there was a Passbook glances. Takes too many presses to get to my Starbucks card. This is one area that needs some work.
 
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