But what would we gain by making such a change? If anything, I think it would be a step backwards in terms of Quality of Life. It'd most likely lead to something like the Starter Syndrome in Pokémon games: You like your starter, so you mostly fight with them, so they end up getting most of the XP and being ahead of your team (and even more ahead of your boxed Pokemon). Then you hit a fight that hard counters your starter, and suddenly you find that your other Pokemon are underlevelled, so you have to go back and grind them (and that's the best case scenario, you might softlock yourself by getting to an area where going back and grinding other Pokemon is no longer possible, and your team can't handle the challenge, so there's no clear way out). And this problem would be exacerbated in BG3 since there isn't really an area of infinitely-summoned enemies for you to "grind" underleveled party members. So you just lock people in to their active party for little to no benefit.