I use one whenever possible. But to be honest, I'm lazy. And in many cases (such as hunting for cute furry critter photos) I don't want to be moving a tripod around. But I totally agree that using a tripod is way better than IS (although not better than a faster aperture lens because you'll never get the
background blur that can make a photo really stand out) but most people don't want to carry theirs around - especially if they don't have a carbon fiber one that doesn't weigh anything and won't turn to ice in the cold. My know my crappy $30 one gets so damn cold I don't touch it in the winter.