Not really. Sovereigns will not only usually be a player's toughest hero but will have tons of nasty magic to fall back on. If you can get a champion with the trait to such a high level that he can actually kill a sovereign then you deserve to win. Would probably only work well against players who shelve their sovereign.
True. But say you have a full stack of well equipped troops led by your sovereign. You're fighting me, whose sovereign and one other random champion has this trait. I go in with the champion first to take a little "off the top" of your sovereign, but my champion is killed, as you suggested and rightly so. Then I go in with my sovereign who also has this trait, and make short work of yours, who has just taken part in a battle.
Meanwhile, your gigantic army is just standing there not able to do anything. Would this be a horribly frustrating loss? I think so. Would it be a satisfying win? Maybe, but probably not. Now, neither of us can really tell without actually playing the game, but it's something to think about, huh?
Edit: I do think that having the code in the game that would allow for target selection when entering tactical combat would be very cool. I would LOVE to make a mini-mod that gave sovereigns the ability to have Highlander-style, risk-it-all duels with an enemy sovereign. Even if it turned out not to be a very fun game mechanic in the end.