TBH the amount of different characters you can choose from matters but the max amount of companions within the party straight up doesn't matter after me. Yes, storywise it helps since you can have more interactions in there and between the characters.
But on the mid-maxing side of things it makes it harder. I made 1 run with 6 characters in BG 1&2 , ended up with a 4 man party very quick ^^. Mostly because BG1&2 had a very silly solution for exping in the game lol.
You mean that the official D&D "exping" system is bad ? BG allow you to play from 1 to 6 character with this "silly" mecanics