I think part of the problem here is people are thinking too much in terms of what is good for them or for the faction. If you look at it from the perspective of what is good for the night elven group, both kalimdor night elves and the broken isle groups incl the nightborne, it's clear that getting toegther with thier people is best option.
From a story point of view, it's by far the most romantic. Look at the night elves as a whole (this includes the nightborne, who are the last vestige of the Kaldorei empire), losing their empire after such an amazing period where htey excelled did so well, but fell victim to over exertion - they tried too hard, got addicted in a way that made their leadership forget themselvs and do the unthinkable. All the grae, beauty progress wiped out.. a hard lesson, millions of lives lost , a great and beautiful empire in ruins, and the world they believed their sacred beauty to perfect instead they end up causing its near destruction.
It goes against the grain of everything they bleieved, it would have broken them if not for the help of the dragons and a nobility that rose up to face the problem. but it left them in a state of emergency for 10k years, forsakking even developing themselve, looking at their birthright as a posoned chalice, using th e properities of the arcane but daring not to wield it to keep the world safe. not because they hate magic. Rememer they hated those who callously and cruelly abused it, but magic made the night elves, and even though they woudln't use the arcane directly for spellwork for a 10k year period, they used its phsyical waters and energies to improve the lands via the moonwells.
To have given up that much and finally attain victory ofver the legion and not ever return to doing the thing they are best at correctly would be a crying shame. Getting back on track to where they were before the addiction came would actually be the most beautiful thing and the promise of a very bright future where for the first time since before the addiction, arcane, nature and divine work together in harmony - it's really fairy tale aspiration of the greatest kind, something warcraft needs. and that's just from a story broadstroke point of view from the perspectiv e of the night elf group as a whole (i.e. kaldorei, highborne, nightborne etc and all those associated with that world.
From a tactical point of view in terms of military, what each can gain from the other, again night elves need nightborne far more than blood elves and night elveshave far more to offer nightborne than blood elves do. Example, night elves though learning are still very low on numebers for the arcane discipline, the few shend'ralar are no where near enough to ake the impact needed for them to be operating in full aracne swing despite their knoweldge, they really need a whole nation of magic users to be aligned with them to give them the magical muscle they need to stand their own against their enemies, it's the only thing enough to match the blood elves and afford the sort of self sufficiency and ability to defend themselves and stand. They have lost a lot of numbers too from WC3, enough to force them to have joined the alliance, and Cataclysm was brutal on them - they were completley outnumbered and over run by a horde invasion, 1 blood elf in Azshara, 2 in desolace were wrecking entire garrisons and military offensives- without a doubt they needed magic which helped them push past their greviances with the past and lift the ban, not to mention now with the revelations about the past, the legions motives and the cure for addiction, not to mention the wisdom they have to use magic in balance and the defeat of the legion including ways to shield magic from them - all the road blocks to it both practically, objectively, psychologyically and phsyically have been addressed. The nightborne and Suramar returning to the fold i think is the kinda boost hte night elves need to make them at least on par with the orcs or humans something WoW has shown they are no where near atm.
As for the nightborne, the blood elves have nothing they can really learn from them, but the night elves do. They excel the blood elves at magical craft, and despite the new and clever ways the blood elves have boosted power, enough to impress Silgryn and Victoire, it is not on the level of mastery and knowledge of the nightborne. Sure a young apprentice like Aethas' ward can learn things from a blood elf, but that would be true for a human because she herself is quite young that's more an indivudal thing than a race thing. the nightborne are the night elves that have an unbroken study and pracitce of arane magic from the days of the kaldorei empire, they don't need anything from the blood elves in that department. From the night elves however, they need the priesthood and nature, the order of Elune's restoration brings a much needed spiritual element which they could get from the light via the blood elves, but the nightborne have no angst against Elune and this is where her biggest and most holy temples are, it makes no sense for a night group to go the human/blood elf light way when Elune is their heritage. And nature, it is nature fused with arcane together that brought salvation to the nightborne from the nightwell dependency and the withering curse. It is balance with nature they require and the mastery of nature magic that the night elves have too is something the blood elves don't have and the nightborne do not, that would also be attractive to to mention necessary to fully restore them.
Furthermore, they are in Suramar, this is Kaldorei/shal'dorei land, it's night elf territory and they are the night elves that hold it, they are surrounded and integrated with night elves too, broken isle night elves fleeing the legion and the nightmare went to find refuge in Suramar their ancient city once the walls went down. Thalyssra opened her arms to them and they helped in the war effort, it would not be good to then take the entire nightborne people and say lets make that horde and deny the rest of the night elves reconnecting with a major slice of their empire, - especially now that htey've started re-embracing their past and they have nhighborne amongst them, and what about the other non-alliance, non-highborne arcane wielding night elves? like the Moonguard?
Finally wouldn't night elves be more concerned about restoring order and beauty and getting back oon track to save the world rather make alliance or horde needs their priority? Restoring themselves would be more important, and that actually means working with the rest of the night elven groups, who may be in the alliance for now, but surely they don't identify themselves as alliance most of them hav elived 10,000 years, the alliance thing is only 15-20 odd years old to them.. what this means is that htey'dbe more concerned with restoring night elven dom and than the alliance /horde thing and it is from this point of view they would reach out to the blood elves, not to schism themselves from the night elves just to bond with blood elves. No, it would be to unify their people againi, and themajor group of them are alliance.
This is why i see the nightborne being night elf ccentric or leaning if not out right being available as a night elf sub-race or alongside the night elves. I can also see them invovled with the blood elves , that too for sure, but it would be very bad if that happens at the expense of the night elves. Unless you think of nightborne as night elves in which case it would just be night elves on the horde side too but have serious ramifications for the night elf restoration story. Especially hfter blizzard has written them going through so much crap.