Don't forget that pathfinding in DF prefers to use diagonals whenever possible, which means that in corners like the one you just illustrated, they actually never step on those node tiles. Plus, players tend to have plenty of doors along hallways.
Still, that's an improvement, you're still only trying to make a linear growth improvement, but it's moving towards fitting the practical problem. One could even try to to turn it into a fractal map where you have larger hub nodes that feed smaller capilary pathfinding nodes, which would be better for pathfinding in labyrinthine dwarf tunnels, but poor for pathfinding in open areas.
Don't forget that we have both open areas and twisting mazes, and we need a system that works well for both.
Look, while it's great that you want to help, if you're going to really improve something, you have to understand the system that DF is using, and try to work out a better core algorithm. Dwarf Fortress apparently uses a modified A* pathing function with a manhattan distance heuristic function, and saved accessible/inaccessible states for tiles.
You should look at this thread, at the very least:http://www.bay12forums.com/smf/index.php?topic=43265.0
and look up the A* and manhattan systems to understand their methods.
DF players (and Toady) know their stuff, and pathfinding has been a known problem for years. If you have some sort of very simple solution to try to solve pathfinding that doesn't require a detailed knowledge of the inner workings of the algorithm, odds are someone else has thought of it already, and it either is already in use, an improved version of it is in use, or it will not work for one reason or another.