Dunno if this was the same in DL, but in DA when minor races spawn late in the game it might as well be a "free uber world" event for the nearest (or most ready) race. They spawn on a class 15ish world, with 1b pop, no ships, no hope (at least on challenging).

In my current large/challenging game I had one pop up in the middle of my territory and took it out within a few turns since I had transports chillin'. Then another popped just outside my territory, 2 AIs go to war with the new minor(without transports - and with nothing to fight against - makes zero sense), and then I take it over in a few turns. So within 10 turns I got two new uber worlds with near zero effort. As a bonus, due to my influence bonuses, the 2nd world also split a rival's territory in half and I'll get easy possession of two of his asteroid fields, since asteroid fields seem to flip very easily. Pretty nice spoils for a "war" that featured two unescorted troop transports.

Seems a bit ridiculous. Well, minor races magically spawning/colonizing at size zero worlds and turning them into better planets than most is already ridiculous. But having them magically spawn just to be galactic fodder is even more ridiculous.