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This is the proof of benefit when you keep it plugged in.
From my almost 5 years MBA. The battery health is still pretty good.
91% is not bad at all.

 
I treat mine more like my iPhone. I use it on battery until it gets low - 20% or so. Then I plug in until it's charged or overnight. The only exceptions are during things like big OS X updates (I'm in the developer program and there are more frequent big system updates) or when I want to make sure it completes a Time Machine backup - usually prior to the aforementioned big OS X updates :), then I will plug it in while it does that.

On a side note, though related: I totally miss having a visual indicator that the battery is charged without having to open the laptop. I want a little amber/green LED on the plug while it's charging, or even built in to the side of the MBr next to the USB-C port. Something to tell me it's done charging.
 
According to the apple battery instructions on the official apple website, it doesn't matter. you have liberty to do whatever is convenient for you in this regard.

Everything else is just an opinion of someone on the Internet, some more informed than others.
 
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