If, as we are occasionally reminded in LotR, "luck" (and therefore "coincidence") is not the random occurrence that it is presumed to be, consider the following "coincidences" of the situation in Bree:
Aragorn is there on the lookout for Frodo, and just happens to find him listening through a hedge outside of Bree.
Butterbur just happens to forget to send the letter to Frodo, which contains vital information as to Strider's true identity.
That letter contains a verse about the "blade that was broken," which Aragorn admits is "not much use," but that "the time is coming when it will be forged anew."
The presumption is that Aragorn must cart this thing about with him all the time, or that it is kept in Rivendell. I tend to think that the latter reasoning comes from lingering impressions of Jackson's films. To me, the more logical presumption is that the shards were kept by all of Aragorn's forefathers. Note that in appendix A, it is said that Elrond "delivered to him the heirlooms of his house" when he told the fatherless Aragorn of his true identity and lineage. It would make more sense, I think, if the heirlooms of the Numenorean kings had been kept by their descendants, until Arathorn died untimely and the two-year-old Aragorn was taken into Elrond's house for fostering and protection. Once Aragorn took up the task as Chieftain of the Dunedain of Arnor, it would seem to me more fitting that he would leave such things in the care of his people when they were not on his person. If so, then it is quite possible that Aragorn, being a man foresighted, sensed that it was time for him to take Narsil with him to Rivendell, where he ultimately hoped to take Frodo and the Ring. He knew that events were moving toward some crisis, and said as much when he stated that the time was coming for Narsil to be reforged. As was already said, he felt his destiny upon him, and acted accordingly -- else there would have been little point in taking with him a sword he acknowledges as useless.
And if he had made a habit of this for a long time, it would have been dangerous. Imagine how Denethor would have felt about Thorongil if it had been discovered that he was toting about the shards of Narsil! Or, if in his travels, Aragorn had been slain by orcs when he was alone, thus either losing the heirloom, or having it fall into the hands of the Enemy (who might then put two and two together and realize that he had managed to kill a still living heir of Isildur). No, I do tend to think that Aragorn was moved to take the sword from where it was kept with his people, so that it would be with him Rivendell to be proof to Boromir (and others) as to his identity, and so that it would at last be reforged. If it had already been in Rivendell, there would've been no reason for him to have it with him, and to me, it just doesn't make sense that he would cart it about out of habit.
Just a bit of post-vacation thought.
