I've got 10+ years or running laptops under my belt and lets just say that for 10-12 hours a day they're plugged in and maybe for 1-2 hours a week they're on battery. I have never had a problem.
A faulty battery is a faulty battery and laptops are designed to run on battery or be plugged in. They're not optimized specifically for batteries - as the OS and APM power mamagement handles throttling things down and slowing down the computer to extend the battery life.
Modern laptops know when to charge the batter and have built in charge controllers. The batteries & technology are smarter to give extended life. Plus the modern batteries aren't subject to "battery memory" as old laptops were.
So keep plugged in when at home and enjoy full speed. Use the battery when mobile. Its terribly unnecessary and often more destructive to constantly wear down the battery and only charge on AC.