I don't see it as a waste of money. We are a long, long, LONG way from having all the answers about the formation of this star sytem.
While it's true that it'll be a long time before all the data gleaned from missions like this will have some practical purpose (if ever), there are potential uses that are very obvious.
This impact marks the first time a spacecraft has made contact with a comet. Much will be learned that for the present only concerns space scientists, but learning how to succeed in this type of mission helps prepare us for dealing with something that we DO know: that comets have whacked this planet before, and will do so again. I don't want to hear the old "but it's part of God's/Nature's plan" nonsense from anybody who sleeps under a roof. That argument reminds me of the public outcry over Ben Franklin's scandalous, blasphemous invention: the lightning rod. His response was similar: the wind, rain, etc. are all part of that, and yet we think nothing of building things to shelter us...Something potentially harmful falls from the sky, you protect yourself. Let Nature have its way with the unprepared- we have the ability to protect ourselves, and I don't mind chipping in a few bucks to help do that.
This spacecraft was not made to deflect a comet, but if and when such a craft is made, it will be possible only because of precursor missions like this. There are, no doubt, techniques, etc. specific to comet- rendezvous missions that don't apply to other flights, even asteroid-rendezvous missions. We had to do it, if for no other reason than to see how it should be done. Sure the gang at NASA and JPL take pleasure in doing these things for their own sake, but that enthusiasm doesn't cost the taxpayers any more than if the control centers were manned by people who didn't get excited about space exploration. The motivation behind it doesn't matter- it's the results and the potential value of the results!
Those nutty government freeloaders with the thick glasses and pocket protectors may just save the world someday.
