You are right that the prime mission will be pretty short. But the satellite will be taking some measurments along the way. Every year the instruments will be turned on to make sure everything still work when we get to Pluto.
It takes lot of energy to get going fast enough to get to pluto in "only" nine years. To go into orbit, it would take a lot of energy to slow back down. If you had enough fuel to slow back down, it is still pretty complicated to put an object into orbit. Take a look at the expensive hardware lob page for an idea on how good we are at that. http://www.bio.aps.anl.gov/~dgore/fun/PSL/
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You are right that the prime mission will be pretty short. But the satellite will be taking some measurments along the way. Every year the instruments will be turned on to make sure everything still work when we get to Pluto. It takes lot of energy to get going fast enough to get to pluto in "only" nine years. To go into orbit, it would take a lot of energy to slow back down. If you had enough fuel to slow back down, it is still pretty complicated to put an object into orbit. Take a look at the expensive hardware lob page for an idea on how good we are at that. http://www.bio.aps.anl.gov/~dgore/fun/PSL/