In the past, I have discussed this question from several angles. I have analyzed sources which discuss the parameters of such protection, and I have discussed the mechanism of such protection. In this post, I would like to discuss a different angle: whether this concept can be said to be of any practical value.
Yated Ne'eman, quoting Rav Yitzchok Zilberstein (son-in-law of Rav Elyashiv), says that it does:
The first thing to do is to learn, Rav Yitzchok added, citing the Gemara (Makos 10) which says: "How do we know that Torah protects [a person as effectively as a city of refuge protects the inadvertent murderer]? For it says, 'Betzer in the desert' [among the list of cities of refuge] and says afterwards, 'And this is the Torah'." “The greatest safeguard for a bus is to learn inside it, especially with a chavrusah,” he said. “For then the bus turns into a beis medrash. If the murderers want to attack a bus, it’s not a bus, it’s a beis medrash.”The claim being advanced here is that the Gemara teaches that learning Torah on a bus protects a person from being killed just as the city of refuge legally protects a person from being killed. But if we look at the Gemara, we see that it is not necessarily saying any such thing.
This passage is raised in the Gemara as a question on a ruling of Rabbi Yochanan. The ruling is that if a rabbi accidentally killed someone, he has to go to a city of refuge (Ir Hamiklat), along with his disciples, in order to avoid being killed by the victim's relative. On this, the Gemara asks that Rabbi Yochanan elsewhere says that Torah protects just like a city of refuge protects, so why should he have to go to a city of refuge? The Gemara presents two answers to this contradiction. First is that Torah only protects during the actual minutes that one is busy with it. Second is that Torah only protects from the Angel of Death, not from the victim's relative.
Now let us analyze the Gemara more carefully. The first point to note is that, according to Ritva, the Gemara's question is not talking about the Torah providing metaphysical protection, but rather about legal protection - that the relative is legally prohibited from killing someone who is busy with Torah, and is himself charged with murder if he does so.
Second, the whole point of the Gemara's second answer (see Maharsha) is that Torah does not, in fact, provide any protection from human killers, only from death by natural causes (the "angel of death").
Third, Aruch LeNer points out that just a few lines earlier, the Gemara discusses the ruling that a Torah student who inadvertently kills someone must go to a city of refuge, and the Gemara does not raise the question that his Torah should protect him. Why not? One of the answers suggested by Aruch LeNer is that the only notion of Torah providing protection is with a teacher of Torah, not a student of Torah.
So, this Gemara does not in fact prove what it is being brought to prove. But there is a bigger issue to discuss here.
One thing that emerges from our brief analysis of this passage of the Gemara is that, as with every passage of Aggadata, there are all kinds of different interpretations, qualifying criteria, and so on. There is no unequivocal claim in the Gemara that someone learning Torah receives protection from being killed by a terrorist.
It's just as well that the Gemara does not make any such claim, because such a claim is quite clearly not true. All such claims about the protective value of Torah and mitzvos - "Torah scholars do not need protection," "Someone on their way to do a mitzvah (shaliach mitzvah) cannot be harmed," "When you're learning Torah, you can't be harmed," might be true in some abstract or hyper-qualified aggadic sense, but are clearly not true in any practical sense today.
"Torah scholars do not need protection"? We saw the terrible tragedy of the Torah scholars who were massacred in Har Nof. In fact, Mishpachah magazine expressed concern that charedim are attacked in proportionately even greater numbers than non-charedim.
"Someone on their way to do a mitzvah (shaliach mitzvah) cannot be harmed"? Some of the stabbing victims of the last few weeks were on their way to davven or to give shiurim.
"When you're learning Torah, you can't be harmed"? We saw otherwise in the tragedy a few years ago at Mercaz HaRav.
Again, you can come up with all kinds of ways to explain how these statements are nevertheless true and why they are not applicable to these situations. Yet it makes no difference. The bottom line is that there is no practical truth or ramifications for these statements.
Now, many people, even in the charedi world, realize this, at least to some degree. That's why, since the stabbings began, many charedim have been learning self-defense, buying pepper spray, and requesting increased army protection. But the problem is that when it comes to sharing the duty of army service, many charedim still trot out these Aggadic statements in order to claim that their learning Torah provides protection and that they should therefore be exempt from serving in the IDF.
There is no claim in the Gemara that a yeshivah student learning Torah provides any protection from Arabs. And the painful facts on the ground show very clearly otherwise. It's time for everyone to face up to this, and to its ramifications.
See too these posts:
Torah Against Terror?
Torah Protection: What is a Halachic Source?
What Is The Mechanism Via Which Torah Protects?
Who Doesn't Believe That Kollel Students Are As Good As Soldiers?
Torah Study and the IDF - A Response to Rabbi Yair Hoffman
Torah, Army, and the Bizarre Chess Analogy
"Rabbis Do Not Need Protection"
(and possibly other posts that I have forgotten about)