I can, but then I’ll have no guarantee that others will see the doc just as I intended.
First example that comes to mind is my snake game, where horizontal gaps were picked to be 6px wide to match font line heights (vertical gaps). I adjusted them for the new font before realizing it was a system-dependent one. Guess my snake game will not look proportional on Macs.
I know it’s an extreme example — it’s unlikely that anyone else will make LCD-like games on Coda as a real use case scenario 
Still it can affect docs that rely on some beautification and branding (e.g. use of emoji, rectangles as custom charts, maybe something else too)