PERSONALLY I think what PoE1&2 (and earlier Deus Ex) did with rewarding players almost exclusively for quests was brilliant. That said, some (many?) people disliked it. Balancing various playstyles becomes trickier, if player can miss out on XP through their playstyle.
I'm not a big fan of filler combat. PoE's problem was, unless they added a bunch of filler combat encounters after their "XP from defeating enemies" change, that it was too much pointless, boring combat. I didn't play it before the change (it was during development/beta testing iirc?) but I certainly wouldn't have wanted all that filler combat to be even more pointless and not even rewarding XP.
BG3 doesn't have all that much filler combat. Quest tied xp earning makes more sense for it. Could do like PoE and have the majority of XP be quest/ discovery/encounter based and only reward small slices for direct combat kills. Iirc in BG2 you eventually had such large numbers kill XP were basically insignificant and only the quest reward XP mattered.
