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

[factotum]

Now, you can define planet as 'everything in hydrostatic equilibrium smaller than a brown dwarf.' The problem with that is the resulting list includes dozens of objects right now (including at least 19 'moons') and will probably balloon to the hundreds in due time. The IAU chose to bestow 'planet' on a far more restricted group, one supported more by linguistic history than anything else.

You can lose the moons from that list easily enough--"Everything in hydrostatic equilibrium smaller than a brown dwarf and which is orbiting the system's primary, not another body". Problem is, it's still way too wide a definition--it includes the asteroids Ceres and Pallas, for instance, which are absolutely tiny.