Holy balls, how many times have we had this conversation? Will you people ever give it a rest?
First of all, the IAU's definition is for technical and scientific discussions/communications. If you want to call Pluto a planet on your blog or whatever, go fucking wild. The IAU neither has the power (nor the desire!) to dictate language for all human beings for all time.
Now about technical language. The purpose of technical language is to provide a common agreed-upon vocabulary that is consistent, precise, and efficient. If you named everything a planet, you'd lose precision. People would inevitable invent a new set of categories for the eight 'big' planets and the other 'smaller' planets. Some people's new terms would conflict with other people's terms. It would be a mess.
On the other hand, if you named the 'big' planets anything other than 'planet', it would lose efficiency. They are the planets that are talked about most often, so it makes sense to give them a short, concise name.
Yes, the IAU's definition of planet WAS DESIGNED explicitly so that the eight 'main' planets would be the ONLY ones in our solar system called planets. There are very good reasons for this and the IAU did its job quite well in this regard.