Well you're making a starter for a couple reasons. One of those reasons is to make more yeast. Another is to get the yeast active and ready for the environment you're about to toss them in. You could multiply yeast with just about any sugar solution and oxygen, but they won't be acclimated to an environment like your wort unless you use wort. This means many of those yeast won't make it and you may be defeating the point of making a starter. But, the more yeast or more active yeast argument is an old one and you'll probably encounter other opinions.