Party lock is a terrible idea.
Just seconding this at every possible opportunity where the comment seems remotely relevant.
Locking your party members and forcibly disallowing your players from experiencing large swathes of the story is a stupid, stupid idea that is only ever going to cause a bad taste.
It does not "Add to replay value" as some like to suggest - it's false replayability, because it's forcing you to replay 80% of the same elements over and over again, just to see that extra 20% that's different, and only the most die-hard will have the stomach for doing that.
Replay value comes form having a story that people WANT to experience over and over again... it does NOT come from arbitrarily cutting bits out of that story so that players are FORCED to replay it to see all the parts. This is not the same as having meaningful, story-changing decisions... those can be great, if done well. This kind of thing is just an arbitrary arc-lock, and it's a bad idea.
We ought to be able to choose our party, yes. We also ought to be able to chose to follow up with various characters as much or as little as we feel like we want to for a certain play through. We need to have the right to take them with us or not, and we need the right to at least try do our best for everyone without excluding others.
In other role-playing d&d games, companions have personal stories and relevance to the game, and if you pursue them, it's content to enjoy. if you don't, you don't, and generally pursuing someone's story in act one is necessary to continue pursuing it in act 2, etc., That's the proper way to handle it, because it leaves that choice with the player....
