A fully charged lithium battery loses absolutely capacity much more quickly than a partially charged battery. Even worse, a warm, fully charged battery will lose absolute capacity at an astounding rate. A battery at 140F will be down to 60% of its initial capacity after 3 months if it is fully charged during that time.[1] If the battery could be maintained at 40% charge during that time it would still have 85% of its capacity after a full year of use, even at the same temperature.
This means that leaving a laptop plugged in will kill the battery in a very short time. The alternative is to let the battery continually cycle down and then charge it again. This isn't a good solution either because the extra heat very well may offset any benefit of keeping the battery at a lower charged state.
All in all, I think it means that a laptop is not meant to be on all the time; unless you are willing to replace the battery at fairly regular intervals.