Even more than precision, you need consistency. You need to tweak the rules, not just the elements, to make a score look good. So WYSIWYG doesn't buy you that much.
Eh, but "rules" only get you so far. There's tons and tons of situations where a human with eyes just needs to make a decision to "make it work." That's where "systems-based" engraving falls apart. It comes with practice and experience of doing the art over and over, and encountering all those little nuances in real scores and figuring out how to solve them.
The reason Lilypond is so good is that two people who really, really knew engraving took the time to codify a lot of those rules.