[yaks_hairbrush]

There are eight currently known bodies (apart from the sun) in our solar system which can tug hard enough to dictate terms of sharing/crossing their orbit. Pluto is locked into a 3:2 orbital resonance with Neptune. Neptune dictates the terms under which Pluto can have an orbit with anything close to its current parameters. That's what it means to be a planet. While there is theoretical space for borderline cases, none exist in our solar system.

I understand you'd like to focus on objects in hydrostatic equilibrium. Pluto, Ceres, the Moon, and Titan are really neat, but something happens when you tell their life stories: you end up referencing one or more of the eight heavyweights. By contrast, these smaller objects just don't get referenced very much when we talk about other bodies' life stories.

The large scale structure and history of the solar system has just 8 known characters that really matter, and that's what the definition of the planet captures.

[dotancohen]

The large scale structure and history of the solar system has just 1 character that really matters, a further two bits of debris that are worth mentioning, and a further six specks that might get a mention in an appendix.