LOL @ clubba!
On my motorcycles, I always replace the battery every 2 years or so. Mind you, they are smaller batteries and bikes are prone to sitting around and not being used for longer periods of time, but I consider the $100 or so spent on a new battery like an insurance on a motorcycle.
On a car, well, not as bad as being stranded on a bike, but still, I think that it is a good idea to replace the battery, especially after my episode with this X5. Looking back, I also think that a couple of other issues (4x4 light coming up and a steering angle sensor replaced under warranty, after the car sitting for a few days in a parking lot), or the alarm sensors inside the car going off (again replaced under warranty) could have been the side effect of a low/dying battery... but that is just my opinion.