The issue with not going for an origin character is (as far as I know) that custom characters don't really have any additional interaction compared to origin characters - something that Larian seem to want to avert with Tav. You don't miss anything major but you're also... just there for the ride. Considering the party lock, you're essentially trading a tailored questline that goes through the most of game for ability to fully customize your looks and get a generic Source ability.
