Yes, dwarf fortress is about a randomly generated world where international conflict can grow up naturally. That doesn't mean that we shouldn't have world gen parameters or raw tags that can force additional violence into the world. It is a game after all.
While generic tags that force constant conflict between two races would be very bad if they were given to one of the big five races of vanilla DF, they would be an invaluable tool for modders and for fleshing out more minor races.
A race with fast reproduction and an intense, instinctive hatred for elves would go a long way toward balancing the current population drifts, for example. Likewise, being able to mod in a bunch of races that always attack dwarves, without bothering to steal their socks first, would be a huge boon to military minded players.
Unfortunately, the simplistic nature of such a tag robs it of both the unpredictability of a dynamic world, and the ability for racial relationships to change over time. Players who want to see natural conflicts arise over time, or racial grudges grow out of genuine slights, are going to feel cheated if we get a tag like that and development ends there.
Thankfully, I find that highly improbable. The presence of a forced military conflict tag wouldn't completely overwrite the core goals of the project, to create a living fantasy world that functions without the need for such brutal tactics, to make it work.
The, "but they don't have a motive!" argument, likewise falls flat. Personally, I don't see the difference between a [CONQUERER] tag and a [BABYSNATCHER] Tag as far as motive is concerned. Goblins steal children because of their beliefs and instincts, but the tag itself doesn't say that.
Someday, I'd like to see the racial behavior tags folded into the ethics system, but until that day comes, I don't see any reason why the tags themselves couldn't become more diverse and versatile.