Notebooks and their batteries

I have a 2 year-old laptop and a 6 month-old netbook. I recently read that if my mobile notebooks are always plugged in to a power source,(and mine are 99% of the time) the battery should not be in the equipment. It read they should be removed and stored away. Is this true? If so, my question, is it to late to remove these batteries and put them away for safe keeping? Are they slowly being drained away plugged into the notebook? Thanks for all your help here!

Thank You! Will read.

Very good first article. It says to take them out when not in use because heat is their worst enemy. My notebook battery was not hot at all to my surprise, however, I removed, labeled and stored both batteries. Thank you!