The FIRST time I replaced my chassis battery, Murphy intervened and chose that moment to strike whichever computer it is that decides when to turn off the under-chassis courtesy light after a few minutes. So, the next time I tried to start the PC, I discovered that my brand new $100 battery was deader than a door-nail. I swore to the Costco guy I hadn't left the lights on. He was skeptical but replaced it anyway. So I felt like a real jerk when I thought about it, and then discovered the stuck courtesy light. In addition to unscrewing the bulb, I also added a battery cutoff to prevent anything like that from ever happening again.
As to the reliability of the battery cutoff switch, who cares? At $10, you can replace it every year if it bothers you.
