It's most definitely NOT a waste of money, and Pluto's classification of a planet or dwarf planet makes no difference. I'm pretty passionate about this stuff, so I warn you that it might get a bit boring, but you started the conversation so too bad!
A designation of planet or not has no bearing on how significant Pluto is to our knowledge. Just because researchers changed the label, it doesn't change the fact that Pluto is constructed from a mass of ice and rock (99% ice). Studying Pluto will give humans information about the beginning of the Solar System and how it started, as well as information about the deeper secrets of the Universe itself. You'd be amazed by how much information you can determine from some pictures of the surface. Because our information of the solar system already includes quite a bit of information, examining pictures of the surface lets us process them and look for clues that will confirm various theories and open up new questions we didn't even know existed.
That's not even including all the benefits of creating the probe in the first place. Going to Pluto is a difficult thing, therefore technology must be created to sustain the Probe's life as it travels through space on its way to the little guy. The technological advancements discovered through the creation of the probe will spread around the consumer industry in due time, and this is a large part of how the general public benefits from NASA programs. How do you think we got things like diapers and Microwaves? Yup, NASA!
