You're really, really confused. Night Elves are emphatically *not* "accepting High Elves back". Back before the Sundering, there were two castes of Night Elves: commoners, and the aristocracy known as Highborne. The Highborne were the ones that dove headfirst into arcane magic. While it wasn't necessary to be of Highborne caste to practice arcane magic (see: Illidan Stormrage), arranged marriages and selective breeding ensured that the power was pretty much kept to the Highborne.

When Queen Azshara, formost of the Highborne, went off the deep end, a chunk of Highborne went along with her and summoned the Burning Legion to Azeroth, and a massive war happened. Some of this bunch eventually turned into satyrs, the rest were transformed into naga with Azshara when the Sundering happened and Zin-Azshari fell into the sea.

However, some Highborne refused to go along with the crazy and joined the Kaldorei Resistance. They fought on the side of Malfurion and the common born Night Elves during the war. When the war was over, Furion and his lot banned the practice of arcane magic because it was deemed too dangerous. By this time though, the surviving Highborne were well into arcane addiction and tried to argue against the ban. They decided to create an arcane storm to prove the point that arcane magic was too potent to ignore and should be kept around, which was the last straw. These Highborne, led by Dath'Remar Sunstrider, were exiled out of Kalimdor because they refused to give up the practice of arcane magic. At some point or another, Dath'Remar got hold of some of the water from the Well of Eternity (general accepted theory is Illidan gave it to him), and when the exiled Highborne came to what is now Quel'Thelas, they built the Sunwell and evolved into High Elves through loss of exposure to the Well of Eternity and drawing on the new, weaker source to fuel their spells. Culturally, they also claimed the sun rather than the moon as a symbol, quit worshipping Elune, etc.

The Highborne that you see walking around Darnassus and the rest of Azeroth right now are not High Elves. They're the minority of Highborne that didn't follow Dath'Remar Sunstrider and his people into exile because they were isolated in far-flung places like Eldre'Thelas (aka Dire Maul) during the War of the Ancients, and didn't get the memo that the arcane was verboten. That's why they still look like Night Elves, because they are; they were never exposed to the Sunwell, never gave up the nocturnal living or worship of Elune, etc. Shortly before the Shattering, some of these Highborne had visions of awful things to come in Azeroth, which is why they made the journey to Darnassus, and Tyrande lifted the ancient ban on arcane magic so that the Night Elves could fight back against the awful things that happened to them (Horde offensives in Ashenvale and Darkshore, loss of Auberdine, etc).

The silhouette argument was only the most obvious one barring the addition of playable High Elves. There's still compelling arguments against it, such as the redundancy of yet another playable elf race when we already have two (one that wouldn't even have a unique model at that).