Though I said in the topic line that I thought this was a stupid idea, I feel the irresistible urge to defend it.
The first thing that people said is that they don't want a multiplayer experience. Hey, neither do I particularly, but this idea has about as much multiplayer as a succession game. Those things produce some of Dwarf Fortresses best stories. Boatmurdered anyone? I doubt that the idea I gave would produce anything quite as entertaining with fortress mode's ability to create hilarious situations. The game, however, has proven its ability to create unusual and amusing stories all by itself.
As to the difficulty of coding such a task as this, I have no real expertise. I've never programmed anything. Well, there was some tiny thing in BASIC back in 6th grade but I think that doesn't count. Still, as I see it, the only real function that would be needed is the ability to export and load every dwarf in the military and their inventory. A pretty small portion of any given game. Then one could start this separate Army mode that would simply check the saves for any exported army files. Two are selected and they attack each other on a simple terrain. Toady said that he would like it to be possible to stumble upon a battle in adventure mode which means that there will be an AI for two armies fighting already existing. No extra coding.
There is one problem that I see and its only fair that I point it out myself. This idea may not be popular enough to warrant the effort. That's why I posted the idea. See how the masses accept it. So far seems none too good. But maybe now that I've gone over the initial complaints, the masses have converted.