In the end they still are just regular companions, bar the one you choose. Didn't BG have custom characters as well for multiplayer with a made-up bio? It just appears that someone in the dev team thought this was a good idea, his personal choice and the rest can not or do not want to change it, like someone's pet peeve. Also not much additional work goes into it. It could work well, it just appears cumbersome to design a system this way and poor character writing naturally interferes with this system more.
I think this is the issue. Don't get me wrong, Larian has pretty good writers but this kind of writing is very complex and you need to write many different dialogue lines to accommodate different scenarios and make these lines work with each other. And I think Larian has much to improve in that regard. The biggest problem with Larian is their tendency to just do things because they like them or think they are cool without putting serious thought into how
