I want CSS to make spacing decisions so that I don't have to, and a system where graphical elements look good by default because they come with appropriate margins and containers adapt to what I want them to contain (e.g. by shrinkwrapping the content or by switching between different numbers of columns or rows) is more useful than a non-system that looks bad unless I keep many tedious ad-hoc specifications consistent.

Moreover,

> Margin breaks component encapsulation. A well-built component should not affect anything outside itself.

Definitely not, margins are an essential part of a component, like a garden around a house, and they make components "usable in any context or layout" (unless the context or layout is stupidly overcomplicated).