It’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.

Also matters a bit what happens to a surface that they don't do anything to. Does a precisely rough surface get too rough or too smooth? Does a precisely smooth surface get rougher in a way that's beneficial?

Could be the case that in-practice this means they just worry less as their perfectly smooth planes get a bit rough.

yeah. what are the effects of too much roughness? may be safer and easier to maintain at smooth than at a specific roughness spec