Not my field at all so this is a real question: wouldn't that percentage depend on the student ? It was my understanding that people respond differently to different ways of teaching (some learn more with visual content, some prefer audio stuff, some are better with books, some respond better to a combination of multiple ways), so wouldn't there be a certain "kind" of students whose favourite method of learning would be through an intelligent tutoring system ?
I know this is purely anecdotal so only for illustration purposes and not as proof, but for instance in my case I know I always fear "looking dumb", even though I know I'm here to learn, so it stops me from asking questions that would make me look like a complete moron. When confronted to a computerized tutoring system (even a basic one such as the Open Learning Initiative courses) I don't have that kind of anxiety so I am not afraid to make mistakes and do the same exercises over and over until I get everything right, which in the end helps me learn more efficiently (or so I like to think).