The whole point of science is that we create better models, more rigid definitions, and hopefully move forward our knowledge base. The scientific community decided that the term planet should meet certain criteria, so that when the term was used people would have a better idea of what it meant.
Here's another way to look at it. Take any piece of rock on Earth, say [ame="http://en.wikipedia.org/wiki/Granite"]this one[/ame].
Not terribly long ago, granite was just a different looking rock. The scientific process created a more through taxonomy so that we could talk about a very specific kind of rock and know what it meant.
Things like how the rock was formed (in this case, it was igneous), what kinds of things made it up (in this case, it is predominately feldspars with 20% quartz by volume) and where it forms (this specific rock is intrusive). Further, we have things like relative toughness, crystal formation, and density to help us narrow down the properties of a very specific kind of rock.
The same with celestial bodies. We have terrestrial planets, planets called gas giants, plantoids (like Pluto), asteroids, comets, moons, all sorts of stars, galaxies, and well, lots of other fun stuff. The more we learn about the outside universe, the better we can make our taxonomical classifications for these items. It in no way diminishes an object to have a more specific definition. Actually, it is just the opposite. It makes that object more unique because we have put it in a smaller class.
Science has to be immune from nostalgia over its definitions.
