The reason the lens designers make lens hoods removable is for compact storage, filter adjustment, etc... It's not because they considered it optional to use. All of the lens designing and testing was done with the lens hood on, and they expect the lens to be used with the hood on.
Lens hoods are like seat belts. Most of the time it may not matter much to your driving whether you are wearing one or not. It's there for the problem situations. With photography some of those problems (flat contrast, desaturated color, flaring) may not be evident until you see a larger version of the photograph.
Some folks say it's too much of a pain to use lens hoods. C'mon, it's a pretty simple habit to learn; my hands do it automatically when I pick up the camera.