Wormie's 2 cents:
It is not a hill I am willing to die on. I think there are far more pressing concernes that potential implementation (or even lack) of flight and spidercrawl.
I do think that what Solasta did with verticality, flight and spider crawl was superb - especially when combined with darkness fighting spiders who could walk on walls, felt really good. Mechanically sold on the idea of fighting giant spiders, something that teleporting spitting spiders of BG3 just don't convey. Having a rogue crawl on walls, or having my mage fly out of danger, was very cool as well.
Even if possible to implement in BG3, such 3 dementional freedom of movement might be another game and progression breaking feature - not something that BG3 needs more of, I think. If flight will be implemented in a way similar to D:OS2 it will be particulaly unappealing considering how far reaching the jump is already. If jump was greatly nerfed and flight would extend it + provide hovering: I think there would be a decent utility of it and would make the game overall feel better. And I must say, that flighers were somewhat annoying to deal with in Solasta, and BG3 at the moment lacks some crucial things like ready action. Not that Larian shouldn't try to implement flying creaters, but I wonder if they could come up with something better then them being unreachable.
Spidercrawl is the more interesting to me, as an ability to walk on walls could be restrictive enough to make it compelling. Personally, ladders being a "teleport" obect, rather then vertical walking has been my petpeve for a while. It feels off how much ground can one seemingly cover when game treats ladders as interactable teleports between planes. I don't know how useful spidercrawl would be in BG3 without serious redesign of arenas - I can't think of a spot that I would like to climb, where there woudln't be a ladder already or a way of jumping up.
