If 9 out 10 of days, the iPhone Air built-in battery is enough, then you only need to use the battery pack 1 out of 10 days. And even in that 1 day, you'd only have the battery pack attached for about an hour (the time it would take to empty the battery pack at 12w). So all in all, in a given 10-day period, you'd only have a thick iPhone Air + battery pack phone less than 1% of the time, and the 99% of the time you'd just have an thinner phone.
I'm not getting the Air, but it's for camera reasons. The battery is a total non-issue.