I like having a well written character either as a companion or as the MC, playing a premade character means you have a better chance of getting that. The only issue that I see really arising from the multiple main character design choice is that it runs the risk of compounding either poor or uneven writing. There are some companions in the old Baldur's Gate games that were very underwritten compared to the others; same for DOS:II, not all the origin stories were as engaging as the others.
