Why (if anything) is a planet clearing its neighborhood important?

[Peelee]

I am very serious about having a problem with planethood changing due to the distance of the object from the Sun. What about a Jupiter+ mass object with a very eliptical orbit like a long period comet?

Is there a particular body fitting these criteria that you would like to discuss? Or are you just interested in the "what if" game (as I call it)?