I've had my dell for 5+ years and the battery still keeps on chugging... (less than half the original battery life though)
I've seen way too many laptops battery fail in 2-3 years... I'm really starting to think that manufacturers are cutting corners on battery manufacturing...
Also depends on how you use them. Batteries are meant to be used. Most people buy a laptop, keep it plugged in for 4 years on a desk, then wonder why the battery that has been in a constant state of charge/discharge for 4 years doesn't work. Take out the battery if you're not using it as a laptop, you'll save years on it.
Using it on battery and letting it get full discharges is actually better for the battery.