I think you just summarized in one sentence why many people hate lawyers. It's not just that lawyers are dispassionate, doctors are also dispassionate, and people don't hate doctors nearly as much as lawyers. It's that you miss the forest for the trees.
Categories are inherently imprecise, and focusing on them exclusively leads one to focus on delineation of experience as the ultimate arbiter. Categories cannot prefectly map reality, normal people know this, while lawyers ignore this. Those vague moral arguments are what matters to moral people, not some outdated belief that Aristotelian categorization somehow can properly capture reality.
Lawyers categorize circumstance and then map the law onto those categories. When the law doesn't map well to circumstance, or when we cannot properly categorize the facts, we have questions of interpretation.
"When does a change in degree become a change in kind?", or "Do we need a law of the horse?" Both questions assume categorization as the problem. But our minds don't work like that. Everything that we're learning through cognitive science tells us differently.
For some takes on this that tell it better than I, here are some book links: http://www.amazon.com/Philosophy-The-Flesh-George-Lakoff-ebo...
http://www.amazon.com/Clearing-Forest-Law-Life-Mind/dp/02269...
- Philosophy In The Flesh [https://www.amazon.com/dp/B001FSJAWK]
- The Political Mind [https://www.amazon.com/dp/B0017T0B2U]
- The Way We Think [https://www.amazon.com/dp/B00AAL62RO]
- Louder Than Words [https://www.amazon.com/dp/B00918JOBI]