There are a couple of reasons not to use it all the time. The most obvious one is it uses up the batteries faster. And you don't want to have it on when the camera is mounted on a tripod as there is no shake to counter-act...unnecessary wear on the gyros.If the steady shot helps pictures not blur, why wouldn't you keep it on all the time?
No, but it can make a beginner feel he doesn't need to learn how to hold the camera correctly and could develop sloppy habits.Does it take away from some pictures?