"Any who wish to learn." And at least one who didn't.

"Maintaining more complex spells takes focus. If you are keeping a shield around a group in a battle, you cannot afford to lose your focus, even if the foe is shaking the ground under your feet. A fast spell requires focus just for a moment, but many of the most useful spells require continuous focus. Mastering focus is one of the keys needed to master your magic. And it doesn't hurt in other areas of your life, being able to focus, either."