Yes a companion can leave your party and can get aggressive towards you. You may start with only one character which I do not mind at all, as you get to replay the game with another origin which is a big win. I also know why allot of people like to start with a party at the beginning but Larian need to make it more sense when starting with those characters at the beginning of the game to the end. "Hey why are you in Jail with me? who are you? other party replies "I'm just a minion that helps you with your quest" THE END.
Though I am sure they said you will meet some companions in jail, and I can agree that when playing the second play through you may know where all the companions are at or already know their back stories with the same companions in jail. That is why I can see that people want to start with a party and get different journey with them instead of finding companions that you did not take or know more about in your first play through.
So yeah I can actually see both sides, though if we start with a party it need to make SENSE and choose which companions are with you that have back stories why they are in jail and so on. Who knows we might have companions or origins thats start from the jail and others you can find in the world (which may not have a origin at all). It's really confusing as Swen said that these companions have origins and playing multilayer they will all start in jail with you and they remove the companion from the world that have the same origin as you.
I know there are some people who would love to roleplay with themselves but I believe Swen say that you still can do that on the AMA reddit. I am more into the developing relationship system and I know most people would not love to roleplay each of the character and decides who gets the bacon or oops I have back stabbed myself...
So is Love&Hate (I am more about in between) will be worth it? Yes because for a better single player experience as you get to choose and make decisions within your party.