As a dev that does a lot of his own design, I’ve never really understood the need to build a full fidelity reproduction of the layout systems a design is targeting. The limitations and considerations involved are deeply internalized and for the most part, I know exactly where designs tend to break and how to account for them. The layout system is effectively running in my head the entire time I’m mocking things up.

So while it’s nice to have tools to help with menial bits like correct spacing, getting every little behavior right in the mockup feels a lot like unnecessary busywork.

Naturally things are a bit different in a team setting, because it can’t be assumed that everybody involved has this level of knowledge/experience, but well… maybe it’s not crazy to expect designers to carry this set of skills, and it’s perhaps not a good thing for parties outside of design and engineering to be able to easily poke and prod at designs directly. Having the design team as a required intermediary helps sanity check changes.