Why would you want to build a wall on top of a floor? Do you ever actually do that in real life?
YES! You must have some sort of base structure to support a wall. It is possible to build an arch or spanned structure which has no base floor for a section, but that requires either special materials, advanced skills, or larger amounts of time. Usually building a spanned wall structure requires a combination of all 3. Having a floor underneath the wall makes construction so much simpler. There is such a thing as a subfloor, and this commonly what both the wall and finished floor are built upon.
You don't build some intricate floor and then cover the entire thing with a wall, do you?
I do when the overall architecture requires it. My current project calls for all real stone flooring and sheathing of the fireplaces. The fireplace sheathing is essentially a wall section, but there has to be a floor for it to stand on. I can cut some corners where the sheathing will be, but for the most part I must make the flooring flow smoothly into the wall. The requires building the floor just past the plane of the wall with the same quality as everywhere else. As the homeowner has had trouble deciding on the final appearance of things it is likely that my final 'wall' positioning will cover quite a bit of my floor work.
That floor is obviously constructed. Could I carve some sort of masterpiece engraving acrossed all of it? Probably not, but I certainely could make many smaller engravings. Some of the stone themselves are near masterpiece quality simply based on being carved to shape.
I might even make a few masterpieces that cover only a single stone. It is possible that someone of sufficient skill could carve a masterpiece engraving acrossed many of the stones. There is no reason to not let a user have a construction be engraved.
When it comes to megaprojects, quickness of construction vs long term value IS important, no?
This really is dependent on the person paying for the construction. Everyone that does any sort of work realizes that there are 3 factors that can be used to guage that work. Most commonly these can be called time, quality, and cost. These are applied in the form of a triangle such that the valuation of each factor is both the measure of a side and its corresspondingly opposed angle (think hypoteneus vs 90 degree of right triangle). Low time and great quality will have a moderate cost, but great quality can be achieved with a low cost and great time. Most often the time and cost are what is considered, and quality becomes the computed result.
What I really want to see in DF is the ability to build multiple things in a single space. There is no reason we shouldn't be able to build a pressure plate on a bridge, or build a workshop over a grate. Some engravings aren't that big, we should be able to put multiple engravings in a single tile. I want to be able to build a lever into a statue that is on top of as grate. Pulling the lever should retract the grate, dropping both the statue and the puller someplace. The statue as an item should be able to activate a pressure plate which causes magma to flow into 1 tile. This destroys 5 masterpiece engravings from 5 dwarves causing tantrums and fun in my fortress. DF just can't do this yet.