The choice of the Half-Elven was given in the
First Age to Earendil and Elwing, and their sons Elrond and Elros, not
to mention the children of Elrond. However, it is possible that,
contrary to its
given name, this choice was not necessarily given because of their
descent. Rather, it could have been because of their Maia blood.
First, to get some proper background, we must examine the Maiar. Maiar are spiritual beings, of the Valar’s order, but a lesser degree. Being like the Valar, they too have the ability to assume a physical form if they will. And once assuming this form, they are also free to shed it. Also, like the Valar, they have a great deal of spiritual power. For all important purposes, the Maiar are divine beings. (Info from the Sil, Valaquenta, the section marked “Of the Maiar”)
Now let us take a look at the Elves. The Elves awoke when the stars were the only light east of Aman. Though bound to a physical body, they were deathless, so long as it was sickness or age that afflicted them. They could, however, be slain, and they could die of grief. Their wisdom grew with time, and after many long years the fire of their spirits would consume their bodies. (Sil, “Of Men”) Yet in the end, it was their doom to fade into the twilight; to lose their power and influence in Middle-Earth. This “Twilight of the Elves” did not begin until the Third Age, when the Numenoreans came back, and Man began to take a more leading role in the guarding of Middle-Earth. However, it was their fate to sail West or fade away.
Next, let us look at some history. Melian the Maia married Thingol the Elf, and together they ruled Doriath. After some time, Melian gave birth to Luthien, who, due to both her lineages was half-elf and half-Maia. Though her Elven blood had made it impossible for her to cast away her body, she did have great power, power enough to put even Morgoth himself to sleep. Other than this, there was no large issue with the marriage of an Elf and a Maia. Both were immortal, and both had a measure of spiritual power. Both were bound by the Ainulindale. The issue did not become an issue until Beren married Luthien, and they had a son, Dior. Here was a mingling of Mortal and Immortal Races. Dior then married Nimloth, another of Elven blood, and they had two sons, who were slain by the sons of Feanor, and a daughter, Elwing, who went on to marry Earendil.
On the other side of the family tree we start with Tuor, who was sent to Gondolin by Ulmo to warn Turgon that his day had finally come. Tuor then Married Turgon’s daughter Idril, and she bore Earendil. After the fall of Gondolin they came to Nan-Tathren. After some time he met Elwing, and they were wed. Elwing bore Elrond and Elros.
Now for the argument. Earendil was truly Half-Elven, and this no doubt played an important part in his being given the decision. But other than him only Elwing had set foot on Aman. Neither Elrond nor Elros were anywhere near. This would mean that Earendil is the exception, rather than the rule.
For Elwing, Elrond, and Elros, the choice was given because of their Maiar descent. Each of these carried the blood of 3 races: Elf, man, and Ainu (Maia). Because of their Mortal descent, they were doomed to die someday. However, because of their Elven and Maia blood, this could have been a very long time. It is possible that Elros may have lived longer had he not made any choice. Maiar had great power, greater than the Elves because the Maiar were divine beings. With Luthien it was shown that this power was hereditary. To have a line of semi-divine beings walking the Earth with an indefinite lifespan would not be feasible. Therefore, when the time came, Manwe issued a choice to those of the line of Luthien. Manwe could not offer another being a part in the fate of the Ainur, his own kind, because only a “pure blood” Ainu could claim this, and only Eru could give that kind of power. Instead, he offered them the choices of race of which they were already the greater part: Elf or Man.
Elwing and Earendil chose to share the fate of the Elves, and so they remained in Aman. Elrond and Elros never set foot in Aman, so they were allowed to remain in Middle-Earth. Elrond chose to be judged with the Elves. It was not until the Third Age that this became a problem again. In about 1000 TA Elrond married Celebrian, and she bore him children. Once more, the issue of semi-divine beings had arisen, and was no doubt foretold, for in LotR we are told that Elrond’s children were also given this choice: to sail with him or to remain in Middle-Earth and become mortal(Appen. A. v, tale of Arwen and Aragorn).
So why didn’t the Line of Elros share in this problem? Elros chose the fate of Men, and so chose the fate of all his lineage. Many generations of Men live and die in a single Age, even of the Men of Numenor. As Men came to power, the Twilight of the Elves drew nearer. As the Elves faded, so did their power and influence in Middle-Earth. Any Elven power the Line of Elros had would fade not only with the Elves, but with the passing of each generation as well. As the line continued, the king married mortal women, cutting the Elven blood in half with even generation. All that was left within the line was the nobility of the blood, with little power. The Maia blood, which was already in a fairly small amount by the time Elrond and Elros were born, was also thinned with each generation, and with it’s thinning diminished the power of the line, though the Ainur (particularly Maiar) were not doomed to a fading as were the Elves. By the time Aragorn was born, all that was left to him was healing, his foresight, a will strong enough to contest even that of Sauron, and the nobility of his blood.
So why was there no problem when Aragorn and Arwen were married? Certainly Arwen’s Elven/Maiar blood would cause the same problem once more. Well, not really. The marriage between Aragorn and Arwen was beneficial for two reasons. First, Arwen chose the Fate of Man. So, as the Elves faded, so did her Elven power, leaving nothing to her descendants after as little as one or two generations. True, because of their marriage, Eldarion had a greater portion of Maia blood, but it was not enough to significantly strengthen any Maia power he had. Second, the marriage of Arwen and Aragorn reunited the Maia bloodline, removing any chances of semi-divine beings sprouting up somewhere. The Maia power was limited to a specific line of mortals, and this would be halved with each generation as well.
So, we come to the last question: What about Elladan and Elrohir, Elrond’s sons? If they chose to be Elves, they either passed over the sea or faded with the other Elves. Middle-Earth’s recorded history speaks little if any at all of Male Elves marrying Mortal Women, so there is little concern there.
I’ve used as many facts possible to show that the Maia bloodline may have played an important role in the choice Manwe gave to Earendil, Elwing, and their sons. Had it been solely because of the Elven blood, as the name given to this choice suggests, the Dunedain in Dol Amroth, who are said to carry Elven blood, would have to be given this choice as well.
References: Lord of the Rings (LotR), Silmarillion
Translated into Spanish on the site of the Sociedad Tolkien Peruana