> but why would you have such habit even when stationary and the charger is lying around?

Two reasons - One, the charger heats up a lot when I keep it on. Of course, you don't feel it unless you are nearby or touch it. Two, in the past I once had the charger destroyed due to a power surge. The Macbook remained fine after that power surge but I had to replace the charger since it stopped charging after that.