From my experience, you will very often see a very intelligent student blow off tests and homework, only to do average in the class, despite having easily picked up the concepts. On the other hand, you'll see someone with average intelligence (or slightly above average, but nothing remarkable) get straight A's because they spend hours every night studying.
Personally, I would rather have the intelligent person who may get sidetracked easier, but is able to pick up new concepts easily than a extremely disciplined person who may take 3 times as long to learn new concepts. I like to think of the intelligent, non-disciplined person as someone whose time to complete a task has very high variance, but averages low, and a highly disciplined, average intelligence person as someone whose time to completion has very low variance, but a higher (possibly considerably higher) average.