Q: Is it advisable to keep a Mac Air plugged in, even after it is fully charged? Does this save it some charge cycles and so, increase its battery lifespan?

So this is pretty much the current thinking:

https://gigaom.com/2013/06/22/extend-the-battery-life-of-your-macbook-no-matter- how-old-it-is/

You'll notice this:

"Apple does not recommend leaving your MacBook plugged in all the time. Furthermore, Apple even recommends charging and discharging your MacBook’s battery at least once per month —"

So those links don't really say that anymore.

Here:

Is it better to leave my macbook plugged in and uncharged it to 0 everyone and a while or use up all the battery every …

PlotinusVeritas

"A lot of battery experts call the use of Lithium-Ion cells the "80% Rule", meaning use 80% of the full charge or so, then recharge them for longer overall life. The only quantified damage done in the use of Lithium Ion batteries are instances where the internal notebook battery is “often drained very low”, this is bad general use of your notebook battery."

Pretty much my current thinking. Reading the current literature doesn't help much: