These features/consequences of alignment aren't coincidental.
- Many deep characters don't fall neatly on the alignment spectrum: This was the cause of the above dispute. None of the alignments were a really good fit for my character, so I chose the one that seemed like the best fit. As a player, I want to play a deep character, and as a DM, I want to DM for deep characters, so telling players to choose an alignment makes it harder to develop those deep characters, and makes it more likely that the DM will tread on player choice (see 1).
-Alignment encourages static NPCS (and characters)
- Alignment encourages shallow NPCs: I can't remember the last time I encountered a LN NPC that wasn't interchangeable with every other LN NPC, and that wasn't a poor Inspector Javert cliché. There is a middle-season episode where the Simpsons go to Florida, and they meet the sheriff, played by Dietrich Bader. His quote: "During spring break, the alcohol companies pay me to look the other way on the shenanigans by springbreakers. Rest of the year? I'm a real hard-a**". That is a really interesting one-note (maybe two-note) NPC that doesn't fit well in the alignment system.
Alignment wasn't introduced into D&D to be a tool for encouraging, or modelling, character growth. It is one of the parameters of a character that a player is expected to have regard to in play: the player of a good character should forego certain means that the player of an evil character is not obliged to; but in return, the player of the good character has access to things (eg friendly healing clerics) that the player of the evil character does not.
Gygax's DMG explains that departing from alignment in play can cost XP (in the form of levels lost) and/or gold (in the form of longer training times).