TL;DR
When you place an item on a matching ghost and hold down the mouse button, dragging should only place additional items if they would also match a ghost.What ?
I have discovered that I can fill-in ghosted power poles by clicking and dragging (or equivalently, running). I assume this is a consequence of the system for running and dragging to place power poles at maximum separation. But this only works for power poles. It should be extended to all blueprint ghosts. The way it would work is, if the first item you placed by click-and-drag filled a ghost, then until you released the mouse button, items would only be placed to fill ghosts. Items would not be placed if that tile did not contain a ghost of the same item with the same rotation.So you could fill a blueprint that contained inserters on every other tile by placing the first inserter and running down the line.
Why ?
Filling out blueprints by hand is a lesson in tedium and aggravation. Especially so when you accidentally place an item with a slight offset, and it clears whichever ghosts it landed on top of.This change would greatly improve the experience for blueprints that contain repeating patterns (particularly gun turret walls and arrays of smelters or assemblers), but still requires the user to pick and rotate items on complicated blueprints that they might have copied off the internet, such as belt balancers. Unlike moving personal construction robots to pre-oil tech, it doesn't change game balance by introducing automatic repair or rapid turret creep at an earlier stage of the game.