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I can think of a few things to contribute to the alignment debate, but I'll restrict myself to the following thought regarding alignment entries on monsters versus the DM deciding how fifty races of creature interact:

Expert DMs are not the only ones running games.

I started playing D&D around the age of 11 or 12.  Maybe younger - my younger brothers and I stumbled across my father's set of five beige books, and got interested.  He wound up getting us the AD&D 2e core rulebook, and we played that until third edition came out.

I was a pre-teen, but still the oldest of everyone who was interested in playing, so I wound up being the DM.  This is where stumbling across D&D in particular was a godsend - having seen myriad other systems, it stands out for having stats and rules for whatever situation might reasonably come up.  The HP of a wall or door, the rules for long treks through the desert, random encounter tables - everything.  This brings me to the Monster Manual.

Those rulebooks were, frankly, my main exposure to fantasy.  Oh, I'd read a few books, but I hadn't even read Lord of the Rings yet and my mythology knowledge was spotty.  The Monster Manual provided hundreds of creatures, few of which were familiar to me - and so I read, and I learned about them.  Of particular use were those alignment entries - 'Oh, this thing's normally good.' 'Huh.  The Fae aren't good or evil, they're chaotic.' 'Goblins are evil, so they basically mess up everything.'  As a new and in particular young DM to the system, having the 'usual' actions of creatures spelled out for me in one or two words was incredibly helpful.  Good monsters were saved for allies, for summoned minions, or for agents of justice when certain PCs danced too far over the moral line.

Those days were Monty Haul campaigns.  My younger brother was better at tactics than I ever could be, so his archer set up ambushes for hordes of stupid goblins and generally triumphed over everything.  But those days were fun because on a whim, with no preparation time save maybe a half-scribbled map, I could run an adventure.  I didn't have to plan ahead because I could crack open the MM, find something level-appropriate to have spontaneously appear, and use the alignment notes to get a start on how it would respond to the party.

I've grown since those days.  As my skills have grown, I've run other systems - ones that require more work of me, in exchange for more freedom of expression.  I've read through campaign settings like Eberron that subvert the concept of alignment, blending it into shades of grey.  I've played Evil PCs in parties of Good PCs, assisting them for entirely selfish reasons.  But I wouldn't be doing any of that if I hadn't started playing RPGs, and I wouldn't have stuck with them without an easy-to-use ruleset.  (Yes, I played with THAC0.  It's not as hard as rumor would have you believe.)

My point is this.  Experts can take the raw numbers and make a new race of morally ambiguous entities to use for their plots at their whim.  But beginners are picking up the system too, and looking to find out what these various fantasy creatures are and how they act.  Give them their alignment entries.  And give them some good creatures for completion and for contrast.  It will start them thinking about how moral shades of grey can enter the equation, further down the line.

Let the new players have the guidelines you're free to ignore.