I mean, let's be honest. "How a creature is played" doesn't come up a lot of the time either. The players come in blasting, and "how a creature is played" is "they defend themselves and try to kill the people trying to kill them" a lot of the time.
Except perhaps during the earliest part of my high school and college days--which was back when 2e was out--I've don't think I've ever been in a D&D group where the players always went in blasting or where what the monster was like wasn't important. My group tried to do the GDQ series, converted to 5e, and gave up because we find that sort of stuff, without enough RPing, to be quite boring.
Alignment for me is a quick short hand to a DM to remind them "this guy is one of the baddies" or "this guy is intended as an ally" or "this guy could go either way" or things like that. Deeper role playing cues are in the full description but those brief reminders can be helpful and take up almost no room in the statblock.
And this is where alignment becomes a straightjacket, particularly when you have racial alignments. Oh, it's an orc, it must be one of the baddies because that's what its alignment is. Nope, can't use them as allies; they're baddies. Oh, it's a gold dragon, it has to be an ally.
-2 points to Ravenclaw for two uses of actually in one sentence.
Good thing I prefer Unseen University to Hogwarts.
No idea what you're talking about here. If the NPC statblock says LG, then I know that NPC is likely not intended as a combat-first encounter and I need to read the description more carefully. If it says CE then I know the NPC is likely intended as a bad guy. Either way, that's a meaningful shorthand which has some uses in my game. The LG is incredibly likely not someone who engages in child sacrifice, and I know of zero published adventures which would use that alignment to represent something like that in the adventure.
"Not a combat-first encounter" and "bad guy" are not descriptions of how to play the creature or what it's like. Also, it's a straightjacket because why should a particular monster always be a bad guy or potential ally? That highly limits what you can do with the creature.
Yes, which is why I say it's a broad short hand and not a full replacement for an entire description. It's a guidepost. Which has its uses as an organizational tool. Some people organize their DM prep without it, and some DMs like myself use it in their DM prep. If you don't use it, that doesn't make it not useful to others.
I can tell you with certainty that many professional writers for WOTC use it in THEIR adventure prep, and were none to please when it was wholesale removed from what they had written without notice at the last second. THEY thought it was something with utility.
And how many of these people prep with alignment because it's a useful tool, and how many prep with it because that's what they've always done, and how many people prep with it because they don't know what to do with a creature unless it has an alignment to tell them that the creature is a bad guy or not a combat-first creature?
The type of armor being warn is OFTEN more important to the adventuring party than whether than particular now deceased bad guy was sinister or sarcastic or plotting with some other minor NPC. Because it was valuable loot, and also might serve as a disguise to kill the next bad guy they encounter whose personality traits they also don't give a crap about and which fry nicely with a fireball.
And again, the type of armor doesn't tell you how to play the character. We're not talking about loot here. We're talking about roleplaying.
Alignment is not a white-room issue. It's a commonly used tool which DMs use. It's far less about theory than utility. How you prep a game, what you do when you glance at a stat-block during a game, these are the things which are relevant to alignment. The theories about the corner cases of alignment can in theory mean are just not important to the practicalities of how it's used in actual games. If some DMs find it useful, then damn dude let them use it. DMing is hard enough as it is. Game prep takes enough time as it is. Adjusting to something the PCs did unexpectedly is hard enough as it is. Don't take away a DM tool because in theory you don't like how some corner cases could work out. Not unless you have a good short hand replacement in mind already.
Can the game take away a DM tool because it has caused countless arguments over literal decades and is often not used properly even within the game itself?