Why you should keep your laptop plugged in (at least on a Mac)

Okay this answer isn't black and white.

On a Mac, once the battery is charged, the electrical system bypasses the battery charging and acts as if nothing is there. At that point, it becomes a standard "desktop" just sitting there, effectively as if you have removed the battery until the charger is unplugged. By keeping the laptop plugged in as often as possible you won't use up the Mac's battery's 1000 recharge cycles.

However, there's one catch: Lithium-ion batteries (the ones found in your mac) need to go through full life cycles to achieve maximum performance. Basically, you want to run through a charge cycle on a say biweekly basis, so that the battery can have the longest possible battery life when it is unplugged.

In conclusion, keep your mac plugged in at 100% as much as possible, but don't forget to run through an entire battery cycle every once and a while.

(Apple recommends discharging once per month at a minimum.)