That wasn't really why it was demoted. Eris was discovered, and it was more massive than Pluto, so a decision was to be made about whether we were going to say there were more than nine planets (and admit we don't know exactly how many planets there are in our solar system), or kick out Pluto and just say there are 8. The IAU made a decision about this, not for scientific reasons but for naming conventions (they have a different naming convention for planets and non-planets).
Keep in mind that though some planetary scientists are in the IAU, most members of the IAU aren't planetary scientists, and my understanding is that most planetary scientists aren't members of the IAU (someone correct me if I'm wrong here). But the IAU is responsible for naming things in the night sky.
The initial definition that was suggested didn't include the criteria for clearing the neighborhood, add a bunch of new planets to our solar system, and say we didn't know how many planets there were in our solar system. Members objected, and went with the simpler solution of just kicking Pluto out and saying there are now 8 planets (the definition with the vague "clearing the neighborhood" part of the definition).
I have to say I prefer the original suggestion for the definition of planet; it does a much better job of conveying to people how much cool stuff is in our solar system and how much we still don't know. As far as I can tell planetary scientists aren't fond of the IAU definition and tend to ignore it (though Alan Stern has lead apublic campaign to get rid of IAU's definition).