No, that's not the main reason people say plug it in, and it's not "people" saying it, it's Tesla saying
A connected Model S is a happy Model S in the card that comes in the car at delivery.
"Battery reminder-- Your Model S has one of the most sophisticated battery systems in the world. An important and simple way to preserve the life of the battery is to always leave Model S plugged in while at rest. This is particularly important if you do not plan to drive for several days.When plugged in, the battery can be charged, cooled, or heated as appropriate, while remaining systems work towards optimization."
Note it doesn't say anything about being prepared in case you need the range, although that's nice too.
Also the manual says to do this. Twice. In bold print.
That's why "A connected Model S is a happy Model S"