Io and Europa don't offer the origin of the solar system information that a comet can. Also, it took four fly by's (three earth and one mars) to get it to the comet.

No mission to Jupiter has gotten there without gravity assist. Unfortunately, where planets to assist are cannot be conveniently changed, missions cannot be launched at just any old time to get a probe to the outer planets. So flights to the outer planets (with out current technology) can only happen when the stars literally align.

Add to that this mission was good practice to intercept a body that might impact the earth (think dinosaur extinction, the Tunguska event, or the recent near miss in Russia), while trips to outer moons are valuable, this mission may have more practical implications. Imagine we find an asteroid heading our way in a few years, what this mission teaches us may give us the knowledge to deflect the object.