It just seems to me like the the best (and honestly only good) option, since you will allways have problems with hotbar in this game, there is simply too much things you can have there ... this way you could have there only few spells you use the most (i mean why even bohter having Air, or Mage armor on hotbar, since you use it litteraly only once per long rest), and the rest could be casted from spellbook (aka. spells interface).
But why not have "spellbook" properly implimented into game UI? The goal is to do what you want to do with as little fuss and as little clicks as possible. BG3 ain't the first nor only DnD game. It's been adapted successfuly to PC for 20 years. The closes comparison is Solasta as it uses 5th edition as well, but I refrained myself from refering to it as I didn't play it myself just yet (I do love those snappy dice rolls - quick and informative). Still their UI seems to do just that and at least to me seems to provide better organization and better information with no manual player management
One point made here is why I feel BG3 plays janky and feels clunky. There are way too many clicks. You need to click several times to Sneak Attack. You need to click several times to use Self Only abilities (clicking once to activate the "casting stance", then clicking to confirm). I disliked this in the DOS games as well and it just slows the game and combat down, in a tedious way. A proper UI, with organized spell casting and doing stuff with as few clicks as possible is a thing that I feel is kind of needed to make playing the game feel like a smooth experience, and not a slog to go through just to enjoy the fun parts of it.
The Solasta UI looks basic and people complain it does not look good/looks like a mobile game UI. While I agree it may not look visually too appealing, it is much better than the BG3 (And DOS it borrowed from) UI. Fancy, nice looking UI does not do much if the controls are clunky and not intuitive and is just annoying to deal with.