rant incoming: I realized I bitterly hate an aspect of Crawl, which surprised me because I am a mild sort.
I hate food.
Why is it there?
I fully agree with this point. For the majority of characters, food never matters. I think it would be better to remove it and try to use other things to limit high level spells (eg glow).
That said, I don't think the rest of the argumentation in your post helps you at all.
It is so plentiful that I end up dumping piles of fruit and rations into my stash. Edible monsters are everywhere. Starving only happens when harpies attack before you are ready. Of course, then you inevitably starve because apparently a hardened adventurer needs to eat a full steak every 20 minutes.
I'm seriously struggling to see how you could starve to death due to harpies. If they're overwhelming you that badly wouldn't you kill them outright? How can your stash be so far away that you can't get back to it, or at least find an edible creature before you starve?
Food does little to prevent camping, because, as i mentioned, it is plentiful. On the other hand, there are functions in place to prevent camping, since layers will spawn out-of-depth monsters when you hang out too long.
This is true.
Food apparently manages and restricts spellcasting. Spellcasting is thus managed and restricted by A. mana, B. Food C. Intelligence and D. Spellcast failure rate. It is so nerfed compared to Nethack, most people seem to use 2-3 spells: namely Haste, mephitic cloud, (fire/poison/frost) cloud. Every other interesting spell is not viable because A. you won't ever succeed in casting it. B. if you do, you will be out of mana C. you'll be starving. again.
Conversely, melee is "managed and restricted" by hitting tab a lot. and sometimes drinking a potion.
You are very very very wrong about the strength of spellcasting. Every level 9 spell is great (except maybe Dragonform, which is merely very fun), and can clear entire roomfulls of enemies with ease. It does take a lot of skill investment to get them castable, but they are so good it's worthwhile. Mana can be restored with a channeling item, Sif Muna's activated ability or Vehumet's passive (which works really well when you're blowing up a whole roomfull of enemies at once). Food isn't that much of an issue especially at high spellcasting and int, but if you think it is you can learn Necromutation.
Why does spellcasting need the added restriction of hunger when it is already nerfed by mana and other factors that make it hard for newer players?
The main answer is "summoning". Without the foodclock and with Sif Muna you can spam Summon Dragon or some other high level summon all the time, and it's pretty difficult to die when you have a massive meatshield constantly surrounding you. I'd agree it's not a very good system though, since you can a) get around it, and b) it introduces lots of pointless tedium for every other character. It'd probably be better to have it affect glow or something.
I also think "glow" is a really interesting limit on spellcasting (but then again I like mutations in general).
Both with the harpies issues and my claim of not being able to cast mosts spell, my criticism mostly refers to the lower levels, which you have to survive before you can cast powerful and interesting spells. The bulk of the lower levels are filled with semi-useful spells you can cast anytime, and useful spells you can only cast a few times: say mephitic cloud. This is restrained by your mana. However it is also restrained by your food.
If you survive to the point where you can can shatter or necromutation, you probably have more food than you will ever need sitting in a stash. Now on the other hand, before lair, before mines, before you even have a stash food is something that is constantly annoying since you cast a bunch of spells, probably end up meleeing or running at whatever survived after you ran out of mana. Rest. Get mana back. Find yourself nearly starving. It is a feature of the game that only affects new characters, and especially magic-using characters and affect them in bothersome ways (thus my rant).
I hadn't considered summoning, you are quite right about that. However, Death knights basically dodge that issue if ghouls, and Liches can summon all they want, and necromancy summons don't disappear like, say, sticks to snakes, while the demonspawn mutation spawns summons at a high rate anytime you are in serious danger. As such it seems like crawl is balanced to handle summoning exploits already.
I don't have a ton of experience on this topic admittedly, but the one time I got a caster deep into the game, I had almost completely run out of food and was eating kill to kill, with maybe 1-3 reserve rations at any given time. It was a combination of not knowing spells caused hunger (and thus spamming spells all the time rather than using a bit of melee) and resting for egregiously long stretches. So in the absence of any hunger-helping items/branches, I assume I'll have to adjust my play slightly to get through the game as a caster. That doesn't strike me as pointless mechanic.
On my early melee characters when I didn't realize hunger could become an issue (since it so rarely is in games) and I didn't carve up many corpses, I would run low on food then too. I learned to carve corpses for food, which is a similar action to praying over corpses for piety. Get used to doing one and you're used to doing both.
How many rations you carry versus how many you leave in your stash is a part of the inventory management game too.
What you are describing is exactly how I play. Storing rations aside, eating whatever I don't sacrifice etc. I just don't see why the crawl experience is enhanced by pausing from the actual game every 30 seconds to butcher a rat/goblin/whatever, eat a chunk of its flesh and wait the thirty or so steps it takes you to get hungry again. Consider the Amulet of the Gourmand and the ring of sustenance exist almost solely to lessen the annoyance of butchering and eating stuff to keep yourself able to cast spells.