I'll add my voice to the fray.
I have to say, one of the most fun games I played in Vicky was just a few months ago, as Prussia => Germany. After unification and achieving my own personal conquest goals, I decided to use my armies and unify the Balkans under one flag. I chose Greece as my puppet.
Long story short, my satellite Eastern Roman Empire reborn would not have been possible without the negotiation option. I fail to see a compelling reason to remove such a game feature.
As several others have already said, I want to have fun playing a game. Recreating Byzantine was
fun. If another player wants to exploit to perform a world conquest in single player, god bless him; he's probably having fun too. Why limit game functionality?
Come on! Let's all have fun together!