When using a Laptop, is it desirable to keep it charged from the Mains all the time?
It is certainly better to have your laptop plugged in all the time. During that time, the battery level will oscillate between ~97% and 100%. This will have a minimal strain on the battery.

Every full charging cycle significantly reduces battery life. A typical li-ion battery will survive up to a thousand charging cycles.

High temperature also affects battery life. Bad heat management of your laptop will take its toll on battery life. Keeping your laptop directly on your lap will shorten battery life. 

Basically everything you do shortens battery life! I have a friend who decided to maintain his battery capacity at a maximum by not using it. He keeps his battery in a shelf and plugs it in once a week. This way, the battery will have its maximum capacity in times of need. I guess it works for some people.