In reply to Nick.

The goal is to move all such settings to theme.json. So, if you want both margin and padding support, just add them both to that file instead of splitting them up. You can use theme.json without building a block theme.

I hope the development team eventually makes this filterable via PHP so that theme authors can choose to implement this via PHP instead of JSON based on their personal preferences. It's one of those things that just seems like such an easy way to win over current theme developers.