I prefer getting locked in so i can methodically play different party compositions with different playthroughs with out feeling like i need to constantly swap characters out for content mid run or i might miss something im intended to see. It is unenjoyable to be uneasy at all times about whether i need to have certain companions in certain situations, and when the game is designed around the idea of locked parties i then can trust the developers have accounted for any party combination in any situation. Likewise, the 'freedom' to shuffle your party is not really giving you more, its actually taking from you, as certain situations might very obviously favor certain party members, causing you to take them naturally, and thus missing out on the content that the off-members might have for that situation.
Some might claim that a solution to this would be to enforce strict party composition yourself, but its not so easy, since a game not designed around strict party has no player assurance that they are not supposed to be shuffling things around as i have mentioned.
