You only need two or three in your party in most circumstances as a rogue can solo significant parts of the game (as of now) and the rest are just meatbags to slow the enemies down or offer targets for an AI which does not know how to prioritize. You need their passives for some checks or a rogue if you are not one yourself. I guess your character, a rogue and a caster for surfaces, speaking with animals and a bit of cc could take down most of the game with patience.
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.
This is an excellent point. History shows that in a lot of cRPGs if you have sufficient mastery of the system you can beat most games solo. The point is that the vast majority of players don't have that mastery and aren't interested in achieving it. What they want is to be able to enjoy a wider variety of companions and companion interaction without feeling like they need to be experts in the system. I'm very much a beginning and need a tank and healer in my party. I happen to like Shadowheart and Lae'zel but I don't want to be limited to them and/or whatever other tank/healer they introduce into the game and one other character. A game like this should be forgiving enough that a person with basic familiarity with cRPGs who learns everything the game teaches and nothing else can get through the game with a feeling of solid challenge while still allowing them the potential to experience the majority of what the game has to offer. Not that they absolutely will experience everything, but that no matter what class they choose, they CAN experience everything. I'm someone with several years of experience with cRPGs, a modest understanding of D&D and I certainly do not think I could make it through the game without Shadowheart and Lae'zel.