J'adore the Baldur's Gate titles. But, as I said, I find games that don't equalize XP to be frustrating and limiting.
The way I see it:
If your party members bring different things to the game (personality, skills, whatever) then having them all level up independently rapidly constricts your play-style, you end up forced into using the same few party members. That means that you can't fully experience the game.
If your party members don't bring anything different, then why distinguish them by level? There's no need to switch either way.
I get that it can be somewhat immersion breaking, but I think the tradeoff is worth it.
Oh, and if the game ever has a point of no return then having under-levelled party members can permanently doom a save file, which is much less of an issue if everyone level's up in lockstep.
"But I don't want to go among mad people."
"Oh, you can't help that. We're all mad here. I'm mad. You're mad."
"How do you know I'm mad?"
"You must be, or you wouldn't have come here."
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