I've heard of Pluto, therefore it's a planet
It seems to me that "planet" is as much a value judgment as a statement of objective fact -- it means "an important object orbiting the sun." So trying to give the word a pureply objective definition will just create confusion, especially if it's promoted to the public as the definition (much like the confusion about the difference between the botanical and culinary definitions of "fruit" or the physics vs everyday meanings of "work").
The value judgment inherent in the term "planet" explains why so many people are so worked up about this issue. We've all heard of Pluto, whereas the other objects that would become planets under the expanded definition -- such as UB313 -- are unknown except to astronomy geeks. Our brains operate on the "availability heuristic" -- things that we can easily call to mind are more characteristic or more important than those we don't know about. The availability heuristic is an outgrowth of the mistaken assumption that our minds are basically passive and objective recievers of information from the outside world. We know about Pluto but not about UB313, so therefore it's obvious that Pluto is important enough to be a planet, and there's something fishy about a set of objective criteria that UB313 fits just as easily.
Then there's the circularity of all this. The reason we know about Pluto and consider it important is that it's included on the canonical list of 9 planets. Had astronomers never tagged it with that label, making Pluto a planet would sound just as ridiculous as including UB313. Thirty years from now, when everybody has been working under the 12 planet system for decades, we'll look back at old textbooks with their nine planets and think "how ridiculous! UB313 is obviously a planet, so how can those morons have left it out of the old books?"