I agree that for the human player (other than late domination or diplomatic pushes) vassal addition is fairly worthless. But in terms of the AI, I have found that warring has become harder due to the vassal agreements between AI. It is much like defensive pacts or permanent alliances.
I have only played a few games of Warlords thus far, but in each one, I was in the process of destroying a particular AI when they became a vassal of another AI, mid-war, which resulted in the new master declaring on me instantly. I have found that I can no longer go to war with as few troops as I have been used to in the past. I used to pseudo-skimp on defenses when I pooled my troops for a major offensive, often leaving only a couple of defenders in my cities. Now I have found I have to assume that I will be waging against more than one AI at a time, everytime I declare war.
Anyway, I am still getting used to the vassal facet of the game.