(Ugh I typed up a whole reply and exitted accidentally now I have to type it all up again)
You bring up books as an example as to why "eating" anything breaks immersion, so the first thing i will point out is that in my view of a roguelike (and in the original rogue actually) and in my actual game, you're not conan, king arthur, or anyone special like that, you are just an unlucky rogue, archaeologist, or lost apprentice to a mage, you are not a hero, you are a rogue, who needs to survive. And who eventually may become the hero, but in no way do you start expecting to be a hero, you are not "the chosen one", (my view on this can be summed up by this podcast:
http://www.roguelikeradio.com/2013/08/episode-77-hero-trap.html), in fact you are very likely to die and not be remembered at all, because the world will die in one year anyway (in my case), and you failed. The next character might be this worlds hero though, but is likely not.
If you are a regular guy, starving in a dungeon, which you (Drunkenly?) bumbled into , and you saw a rat and no other food, you would eat it, you have no other choice, so therefore, the player in my game, (and I think in most), should be able to use the resource that is that rat (or in my case a proccedurally generated vermin creature) to feed yourself, or make weird gurgly potions. If the need arises. (With associated (Vomity) punishments for overindulging in unsafe to eat vermin)
Also my game is a "dark fantasy" game so being rather disturbing (But funny) is part of the fun.
Second point, You mention that the player is still doomed, if they dont have the item they need, this is very true, (and lets point out that by magical, I mean anything enchanted ( an amulet of fire resistance works just as well in the case of an ethereal, but this is by no means, the only example of improvised weapons, in fact you came up with one)), but, this extra ability to wield anything (and as I stated , its not really hard to program if you pre planned it and use polymorphism), is an extra option the (a whole world of options, actually) the player didnt have before. You are more doomed in a standard roguelike. I look at roguelikes as an opportunity to problem solve, and therefore, having more options to solve problems is nothing but a good thing for the player. (even if it feels like its a one-off problem, its still better then always being doomed in that situation) (Of course I also have other solutions for this problem, but I can talk about that some other time, I cant reveal all my mechanics here!)
If a player has no way to escape, they have nothing to teleport,and are essentially doomed, then to be honest, that is boring, the player could potentially survive, if he has an amulet (any magical item). AND MAYBE JUST MAYBE they could survive, when they figure out that amulet doubles as a magical weapon. Much better and more interesting stories come out of this then just limiting the players options. Also players when put in stressful situations forget when they have the solution for a problem alot of the time when they die and realize they could have saved themselves, thats interesting and humerous (usually).
(also my ethereals use a d100 system for ethereal-ness, essentially it is an extra check before a dodge check, that checks, "oh this creature is 50 % ethereal, therefore, every time you hit with a non-magical item, there is a 50% chance it will pass all the way through, so it isnt a 100% you lose thing)
Also I never want to force a player with my mechanics to face death in an impossible situation (I was a jerk and added a 100% ethereal creature early on for example), that is punishing, not interesting, (I have to be especially careful about this because all my creatures are procedurally generated) I want the player to always think they might be clever enough to get out of this by providing more options, more opportunities for that amazing- rogueliek problem solving buzz. (This is especially important to me as a designer)
About consuming rings, In my roguelike , rings are a bit more rare, and powerful then the standard (thats how they are planned anyway), they are magical artifacts you can find and identify, (as in several roguelikes), but let say you find a ring that seems useless, at the time, it protects you from poison, but there are no poisonous creatures around and your inventory is full of items you find more useful at the time, in a standard roguelike, you probabbly drop the ring, in my roguelike, (with this mechanic), you have another reason to keep it around, because it is almost as useful as a potion of poison resistance in this situation, and the effect of consuming it is obviously
temporary since making it permanent is clearly game-breaking, also as a side effect of my system, since a ring is a small item, it can be crushed, and used to brew potentially more powerful potions, then a vermin for example. Suddenly You have 3 more options, more if you have the stuff to brew potions, then you would in a standard roguelike, again, this is only a good thing to me.
Also, you have another layer of decision making, , should I just eat this, or wear it, or perhaps use it in a potion, or maybe throw it somewhere?
(also keep in mind that due to polymorhism, amulets, (if I make them considered" small", have the same set of options by default.
Thats my thoughts anyway! I should probably get off my soapbox and post this on my blog XD, it is at that point now.
Loving the conversation also, its a great way to get some feedback on my ideas!