[avereveard]

"A programmer does CSS"

Rules without margins are going to be a mess. For one you can't negative margin the padding out of the first titles in a section and other useful negative margin pulls.

Without margin collapse, you're going to need to handle first and last paragraph so they don't have half margin from their container.

Margins are respected across the elements hierarchy, which is especially useful if you're reusing components in react like systems; neither nested paddings nor grids aren't suitable do that.