Oh for crap's sake, you guys have no clue. We impacted a washing machine sized impactor in to a comet and made a nice little plume. It didn't make one difference in its orbit or affect it's structure.
We've been impacting things on the moon for years. Take the Pioneer space craft that took some of the first close ups of the surface. Then crashed in to the surface doing 10's of thousands of miles per hour. If they had been watching with telescopes at the time they might have seen some nice plumes then.
Take in to consideration the moon is 1/6 the gravity of earth and has no atmosphere and the plume from the impact will be spectacular indeed. The mission is two fold if you read the facts carefully. One is to get a lunar orbiting satellite in orbit to look for future landing sites. The other is simply putting a junk booster used to get it there to good use instead of becoming space junk. The "Shepard satellite" is the brains and cameras used to get all of that to the moon.
So why not take advantage of all that? Knowing full well you can kill two birds with one stone makes it "COST AFFECTIVE" to just toss in a few extra sensors in the Shepard craft and watch the fireworks. The impacting craft will for the most part vaporize due to the energy expended on impact. That in turn will vaporize some of the lunar rock and dirt and that is what they will be looking at.
Finding water or it's components is the single most important aspect of being able to return to the moon for long term missions. Finding and being able to get to it EASILY is paramount to the missions.