Two things on this... First, +1 to the idea of building walls on floors. -1 to deconstructing the floor if you do it.
Instead, I would like to see walls build on floors be stronger than walls not build on floors. I could be mistaken in this, but the way that I envision each cube of space in DF is that it actually consists of 6 face spaces and a center space. Ceiling/Floor/Four Walls/Center Space.
Digging into a room on the same z level removes one Wall space from the room you are leaving, and one Wall and the Center space of the room you are going into. Channeling into a room removes the Floor space in the room above along with the Ceiling and Center spaces in the room you are digging into.
Of course, I know there are issues with this. Smoothing the Wall or Floor space does not remove these spaces, it simply smooths them, and does not consume the tile. Building a floor consumes only the floor space of the tile, but prevents some, but not all, other constructions. While building a wall space consumes the whole tile. The only sense I can make out of that is that Dwarves like to build their walls really thick.
But, back to the topic at hand, smoothing the floor and constructing a floor should give you incremental bonuses to the strength of your wall against the attempts of building destroyers. This gives you a choice between trading off using fewer materials for the same construction or having a stronger construction. It would also add some strategy into what is built where. For example, building your outer walls you might build on top of floors to make them stronger, while inner walls are build on smoothed or bare floors because they do not need the same level of strength. Of course, construction time would scale according to the number of steps you take to complete your walls, so you end up spending time and resources and getting durability in return.
And since this thread is on the topic of walls, I would like to offer up two more suggestions regarding wall strength.
First, walls that have walls adjacent to them on more than one side should be more stable because they are being supported. In this case, having walls two or three tiles thick would have and much higher durability than a single tile wall that is more than just the sum of it's parts.
Also, this could be applied across Z levels, so that building foundations for your walls increased their durability as well.
So, for example:
[] = base wall defence
[][] = Each wall would get +1
[][][] = The outer walls would get plus 1, the inner wall would get +2
[][][]= (smoothed floor below)The outer walls would get +3 , the inner would get +4
[][][]
==== = (Constructed floor below) The outer walls would get +4 , the inner would get +5
Anyway, just a thought.