The issue with companions in game like BG is that they never really feel like a true part of the story, but rather like optional content - even if the main plot is what makes the player meet them. I mean, Imoen is around sure, but we can just tell her not to come. Same for Khalid and Jaheira - you meet them as part of the plot but that's it, somehow. Companions in BG2 were a tad tighter to the PC ( Imoen served a bigger purpose this time around ) as far as I could see, but still, you could bring whoever you wanted based on personal likings.
The biggest issue with party-based cRPG is that party members rarely really matter.
It's not a surprise then that people would like to just dismiss them entirely. Hell, sometimes their little problems completely contradict the main plot, like, "help me reclaim my castle!" "yeah but I'm kind of on the clock here, who knows what the bad guy are doing to my childhood friend..." "I won't like you if you don't help me" "okay then". Creating your own party ensures that you can optimize your group, and a lot of players like optimization/theorycrafting in their games. You will avoid being annoyed with petty irrelevant problems if you don't like that or feel it's out of place. Not to mention you may not get to choose the starting stats of the characters, prompting you to possibly choose between a char whose story you're interested in, but with bad starting caracteristics which would give you a handicap ; or a character with the exact attributes you're looking for, but a story that you can't even care about. Bioware is very good at forcing you to bring along characters you don't like because otherwise you would suck at fighting...
Of course, the intended goal with DOS2 is to make the party members relevant. However I trust it should work either way, since it is theorized for now that the party members will get their origin stories from the pool of available stories minus the one you chose for your own character. So, hiding the dialogues lines or making them available to the player, there's not a big difference. It can be even funnier to find out that by choosing solution X you really did put character Y in a corner, and try to solve that - or just get along with it.
TL:DR ; some people like creating a balanced team ; DOS2 should not suffer from the "handcrafted story for specific party member" like BG/lots of cRPG ; it's just a matter of enabling people to create as much characters as they want to and giving them full control.
The Brotherhood of norD is love, the Brotherhood of norD is life.