I disagree. Motivation will keep you reading the material, but that alone will not make you remember it that much better.

When you're motivated about something you can practice, you may remember more by doing it (which also acts a spaced repetition). But good luck trying that with, say, astrophysics or macroeconomics.

When motivated, you're also more likely to pick up other books on the subject, which is another form of spaced repetition.