I dont think 6 party members are a direct part of the baldurs gate identity.
I however thin kthat 6 party members are a superior number of Party members to 4. Flat out.
Its a tactical RPG at this point, and its 5e.with 6 party members its ok if 1 or 2 are a martial who doesnt do jack shit.
This is my biggest reason for wanting six as well. I want those extra few slots for one or two characters that don't really contribute to the "power" of my party but who I want in the party for roleplaying reasons. With four, this is simply out of the question, and you are forced to be completely technical, even a min-maxer, in how you set up your party.
Take for example my complete aversion to having a character like Astarion in my BG3 party. I want to play good-aligned, and a vampire spawn is just a huge 'no' for me. However, if party-size were six, I might have my preferred party all set with five of those six slots, and then say to myself: "Hmmm. There's no way I would normally even consider taking Astarion along. But he seems like an interesting character for story/quest/RP reasons, so I'll let him tag along with me for those reasons, just to see how things play out with him." But in a four-person party, there will never ever be a spot for someone like Astarion no matter how "awesome" a character he may be. This is exactly how I've often included Viconia in my generally good-aligned BG parties.