I think Party Lock can range from terrible to excellent . . . depending on how its written into the story.
If you just get to the end of Act 1 and the game prompts you "who do you want to continue the story with" . . . well that is terrible.
But if Act 1 ends with . . . dramatic events at Moonrise Towers unfold where Companion X decides that siding with the Absolute is the best option, betrays you and becomes a main adversary for the next acts. Companion Y heroically dies saving the party during said dramatic events. And when you get to Act 2 you discover that there are two threats that have to be faced at the same time . . . so Companion Z leaves to take care of one of them (and perhaps shows up again at game end). Those events effectively thin out the party down to a 'lock' while adding greatly to the story narrative.
It all depends on how Larian handles it. (and no I'm not confident they would do a good job of it!)
Events like the Ashly/Kaidan choice in Mass Effect are some of the most memorable parts of RPGs.
I actually like the idea of getting my party thinned out to a core group eventually. Often in games I don't know what to do with all my companions and it feels wrong to just leave them sitting around at camp. I always wish there were background tasks I could assign them to or something. I usually do not recruit all companions for this reason and to fit in with roleplaying (for example I've played Dragon Age Inquisition 4 times but every one of my Inquisitors has refused to recruit at least two companions).
