Before we started on development of dominions we played some VGA-planets. IIRC it was possible to capture and reproduce other players ships, which led allied players to combine their best ships making them way more powerful and less diverse than we were comfortable with. HOMM suffered from a similar problem. After a while you didnt have your own best troop as best troop, but some other players best troop.
We wanted to avoid that to keep nations diverse. In dom-ppp and dom2 there weren't national summons to the same extent and mages had more random paths breadth so eventually every player had access to every spell and the most powerful units anyway. We wanted to limit this in dom3 keep the unique feel of nations longer in the game. In the late stages of the game differences are less obvious, but I think we have succeded in making the nations different enough not to be a bother anymore.
I am generally in disfavor of symmetry and equal opportunities
If it is possible to aquire someone else's traits, the game quicky devolves into a boring game where everyone just plays the same nation and it will be possible to find an optimal strategy. Diversification makes optimal strategies tied to each nation and is thereby varied and you will need creativity and clever thinking to adapt to the current situation. You strength will depend on who you face on the battlefield.
Hmm, wonder how that came out
I think I was a bit unstructured.