The problem with stealing designs from other factions is 2 folds:
1. Blood traits, some are just very good (different), and can alter the design by quite a bit (trogs)...
2. Unique Equipments... should be self explanatory. Better bows, axes, swords, no armor, etc...
You would have to make some intelligence when coding this part I know, it will take some time and some advanced thinking, it was also just a cry out to actually put in more unit designs in the core game in which I already said the game needed before release
I hope for some sort of unit design pack, while I agree a community pack might be difficult to handle for new players, just make it so they have to go and download it off the forum to start with. The reason I put the idea for the AI trying to leech off designs would be, sometimes it might even field a few poorly designed units, but it would feel more alive while trying to convert your great idea into its own units.
It would take some time to code up, that is true, if the devs can find a better use of they're time I at least hope they see this note, and patch the game with 6 more unit designs for each faction, preferably some unit designs that aren't rubbish. (I very very rarely use the predesigned units, have used a "Militia" in 1 out of 20 games, and a predesigned scout in 1 out of 10 games, and thats about it, I always find most designs lack traits, or focus, and some tactics are completely avoided in the predesigned unit section.