I agree that it will add atmosphere to some situations, but it will not affect game that much. I will forget about it after 2-3 hours gameplay, only if the weather conditions do not affect the fight, for example, on the water you can slip or something like that. Then after 10 hours I will hate it xD.
The game has more significant factors, and by the way, characters are one of those factors. And this is about any character, not just Astarion.
It's just that I do see a lot of complaints about the day-night system in each topic, as if it would really make a difference. But apart from the atmosphere, it probably won't affect the gameplay in any way, and you talk about it as if it's a mortal sin of Larian and spoils the whole game. It's a little... well, you know... ghh.
It will not affect the game that much...for you. Well, for many players atmosphere is a huge part of a RPG and it was a huge part of the original BG games (I would argue this atmosphere is tangibly missing from BG3 currently). If a lot of people are complaining about the lack of a day/night cycle then maybe that's because for them it is an integral part of immersing themselves in a fantasy world, so yes it would make a difference though clearly not from a mechanical gameplay perspective.