Not really sure what you are exactly trying to say, but let's be clear of one thing upfront: wanting a six-members party has nothing to do with the idea that "you NEED it" to beat the game. It's about enjoying the variety of characters, builds and possible party compositions that comes with it a lot more.
Not to mention being able to carry on more companion questlines during a campaign.
The last point is viable I guess.
I found the majority of companions in Baldur's Gate 2 (in 1 they had a lot less personality) boring or badly written even in my teenage years so I never cared much about them and some people are just getting on your nerves. The Enhanced Edition is even worse. The only rpg where I love the companions is Planescape Torment. Mask of the Betrayer is a close second though. I guess being a Pen&Paper player dulls a lot of enjoyment for me because pc games never reach up to the experience with your real life group and then shallow or bad writing ruins it even more. If the evil characters would not be stupid evil (which only the shar priestess has displayed so far) I would care for them more. If the the good characters would be multi-dimensional I would care for them more. That is the problem with DnDs alignement system and why it is ultimately bullshit. If you want an evil character he should at least be more like Bayaz from Joe Abercrombie's books (First Law triology etc.) or even better Black from A Practical Guide to Evil.
At this point a premade three char group offers more depth because you can imagine their banter, be it witty, playful or funny. But I guess that is the roleplayer in me because it puts my imagination above the failing character immersion with badly written chars who I would not take along me unter any realistic circumstances because they are shallow, stupid and unimaginative.
Solo kill list: One stupid evil (displayed alignment) Githyanki warrior, a dark grove, a Zhentarim hideout full of "experienced" rogues, half a Goblin camp including two hobgoblins, two minotaurs, a bunch of underworld monsters and their wannabe sorcerous overlord