The question isn't whether Frodo can use Sam's rope, it's whether Frodo gets +1d when he uses Sam's rope. I find it surprising there's so much debate about this, though one sentence in the rules would have made it a lot clearer, admittedly. If everyone in the company can use everyone else's Useful Items and gain the corresponding bonus die, it's trivial for a company to game this so that everyone gets a Useful Item bonus die on almost every action all the time, which basically just means you threw away the value of Useful Items, part of the value of Standard of Living, and you just dropped all the TNs. The only sustainable interpretation is that everyone can share items (when appropriate) but only the owner gets the bonus die when using the item. But I feel sure that people will create overly complex answers that add a layer of bureaucracy so they can allow sharing and then rein in the resulting inflation problem partially, instead of just taking the simple answer.
I agree that's the only "sustainable interpretation" given RAW. But it does require a fair bit of hand-waving. The only real answer to "but why can't I?" is "because it's the rules." Beyond that the LM and player(s) will have to concoct a rationalization, such as the examples offered in this thread. E.g., "I don't share it", "It takes practice to get the hang of it", "It's really more of an emotional benefit, like a security blanket", "It fits my hand perfectly", etc.
I mean, it's really the same thing with magical item "attunement" in D&D. It exists for purely gamist reasons, and knowing that, we accept it. But at least there you have the "because magic" quasi-explanation. Useful Items are not magical, they are just...useful. So there's no in-game explanation (such as Attunement) to rationalize the mechanic.
This makes me ponder that there's a parallel to the difference between "narrate-then-roll" and "roll-then-narrate", which is how I would often explain 1e. That is, in D&D you might say, "I use my backstab ability" and get a bonus when you roll, whereas in TOR you would roll some Tengwars and choose to narrate them as a backstab.
Likewise, there is (in my mind) a difference between rules that have no basis other than 'balance' (used broadly) and must be rationalized with narration, and rules that have an in-game explanation.
So in general I'm ok with players/LM needing to come up with their own narrative explanations for rules. E.g., martial abilities with limited uses have been criticized as being...oh god can I remember this invented term...oh, right, "dissociative mechanics". Meaning a rule such as "You can use your whirlwind attack 1/day" is somehow immersion-breaking because if a warrior can whirlwind once, why can't they do it twice? But I, for one, am totally comfortable with narrating this limitation: "Well, positioning has to be just right, and that rarely happens. In fact, it tends to happen no more than about once per day. Funny coincidence, huh?"
The thing about Useful Items is that there isn't any such restriction: it is contingent upon the LM/player not only to come up with reasons why a Useful Item cannot be used, but also what the limit is. It's yet another case where I admire gamers who can put the narrative over self-interest to impose such limits on themselves, but I ask myself why it's necessary to require that.
As an illustration, let's theorize another mechanic: every time you use a Useful Item there is a chance (derived from the skill roll itself, or because of a separate roll) that you cannot use it for the rest of that Adventure Phase. (I might go with once per adventure, unless you roll at least one Tengwar) There's no explanation for *why* this happens...so some folks might call it "dissociative"...but that invites the narration: "Sometimes the head comes off the hammer, and I need a workshop to fix it." "When it gets dirty it's no longer as impressive." "Aye, it's the family curse at work. I'll need a full day in the kitchen to break it." Whatever.
What you gain is:
1) As I said upthread, there's less incentive to pick an item that is most frequently useful
2) You can share the item (you could even increase the chance of it "breaking" when somebody else uses it)
3) There's no need for debate/negotiation/disgruntlement over when, or in what situations, the item can(not) be used.