Another pillion here.
In the winter, bikes are cold and you get wet. In the warmest weather, you still have to wear protective clothing and helmet (and if you don't, you're an idiot). You can't carry as much luggage on a bike as in a car, and you can't take your pets on most of them either. Also, too many drivers either see you as a legitimate target, as the devil incarnate or just don't see you at all. By definition, a 'bump' means you're more likely to be injured or killed than if you're in a car.
On the other hand, they can be cheaper and cleaner to run than cars, don't present so much of a parking problem and enable you to bypass city traffic.
They also mean that the woman in front driving the red MR2 is more likely to pull over and let you pass, won't slam into your back wheel at the lights and will happily let you out of that junction. ;-)