We all know the biggest problem with constructed walls. They can't be removed by mining. This has nothing to do with ACTUAL subject of this thread (that said walls can't be engraved), but it's true nonetheless.
Now then, walls are either built of raw material or blocks. Blocks in turn are produced by a mason. Mason's can produce assorted levels of quality on their items.
I think it's reasonable that a block used in construction of a wall would result in a constructed, engraved wall, with the quality of the engraving being less than or equal to the quality of the block(so ideally an artifact block would be a masterwork engraving). It gives blocks a purpose other than just being able to store stone in stockpiles and the occasional well.
The decrease in quality levels is to represent that the engraver/mason is working with slightly more difficult materials (i.e. a piece of stone that moves when you push too hard) than just a natural wall......
Also has anyone ever wondered about how dwarves don't use tools to engrave? How must that look?
....
*The dwarflings watch Urist McElvesInFetalPositions, having just drunk 4 whole barrels walk up to the wall and observe it, the old master spying out the image hidden within the rock.After ages seemed to pass the oldster cracked his knuckles and rubbed the wall and in a slurred voice said "Right. No one sesh tha' about my mother. I'm going to beat you 'til..." here his eyes took on a far away look. "...'til youz is dwarves and elves. With the elves on fire. And da' dwarves laughing. Just like yesherday before lunch." Then he drew back and slammed his head into the wall, and chips rained down. The older of the dwarflings nodded, seeing a true master at work as Urists head drew back again. One day, he hoped to train under Master Urist....*