I don't see the point of playing someone else character in a D&D-type game. Maybe for a one-shot, but not for a full game. Even less when it puts me in the shoes of a companion.
Chances are far higher I make additional playthroughs with a custom party than playing an origin. These origins are far too fleshed out. Its like playing Triss or Zoltan in the Witcher - its neither your story nor do your have the freedom to really make it your own, since your background and big moments are already laid out.
I haven’t played the Witcher games, so forgive me my ignorance, but wouldn’t it also be like playing Geralt, too?
To me the difference is, like playing Commander Shepard, the game is focused on this character. Actually the whole story of the Witcher franchise as far as I understand it (didn't read the books) is based on him, And I wasn't a fan of it. This was the main reason why I didn't play any game from the series until I was bored and decided to get Witcher 1 & 2 - and again I didn't like it in the beginning because I was playing someone else character, who I had no connection to. But the Witcher is bringing the stories around Geralt to life and he is the hero of those games so there is only his perspective to see them. As much as I dislike playing a predefined character it was the execution of the game and story that made me care about it. Even when you play Ciri in the third installment its just for short parts telling her story, its not taking away from Geralt, but enchanced the experience. Still even though I enjoyed making all the big and small decisions in that game for me it will always be Geralt and not my character.. Still it has a very different feeling than going to BG1 and suddenly play as Imoen the whole game or Minsc or Jaheira.
Looking back at Shepard in Mass Effect he did have a background, but it wasn't anywhere as big that starting the game you had a feeling you missed out on key elements of his past, so you experience his/her story right from the moment that it becomes interesting. Same can be said of Dragon Age 2 or Inquisition.
Dragon Age Origins had 6 origin stories - but they don't run together, even if cannonically they all happen just only your character survives. And again you are there making every character defining decision yourself, not an author who puts you on a railroad for the key moments for your character development.
But the biggest issue for me is that you are simply playing your companions in DOS2 and now BG3. So not only are you using some else's character without any real reason to it, but your are also spoiling the companion's background for yourself.