I've only read to The Gathering Storm, so please -- NO SPOILERS PAST THE END OF KNIFE OF DREAMS!!
The Forsaken infighting has been one of the most engrossing subplots thus far in my eyes, mostly because we by necessity only see it in brief glimpses or by the shadows that it casts. Though sometimes the Forsaken strike me as incompetent peacocks who do nothing but sip punch/mulled wine/brandy and sneer while muttering "so-called Aes Sedai" and chopping naked people in half with gateways to annoy Graendal, at other times they're so remarkably good at their jobs that I can't understand how anyone on the side of the Light has a chance (Rahvin's utter domination of Caemlyn, Sammael taking charge of Illian, Semirhage managing to cause massive civil unrest and murder an entire royal family off camera in Seanchan, everything that Lanfear does prior to TFoH). I had thought, up until aCoS (?) that the position of Nae'blis was wholly up for grabs, and that any remaining Forsaken (except maybe Graendal because what the blazes has she ever done besides jiggle at people and tag along with Sammael on his day trips to Kinslayer's Dagger) had a solid chance at snagging it.
Then the DO pulls Moridin out of his back pocket and goes, "nah, never mind. This guy? He's the guy."
What? Really? The reincarnation of Ishamael is the one who we're going to go with?
Let's run through Ishamael's accomplishments in the series prior to his death:
TEotW: he gives Rand and co. some bad dreams and utterly messes up at the Eye of the World.
tGH: he gives Rand and co. some bad dreams and gets chopped up by a farm boy who's still trying to figure out how to swordfight.
tDR: he gives Rand and co. some bad dreams and gets singlehandedly wasted by a larval Dragon Reborn when his back-up got punk'd by Moiraine.
I know I'm skipping over everything else Ishamael did - causing Artur Hawkwing's downfall, leading the Trollocs, influencing every major evil event from the Breaking up through when all of the other Forsaken were freed [also worth noting Ishamael only did all of that because the rest of the Forsaken couldn't] - but is that enough to make up for his nonstop failure throughout the first few books? We see with Lanfear/Cyndane/Moghedien/Semirhage/Asmodean that it's uncommon for the DO to reward failure, or even give more than a passive second chance (Osan'gar and Aran'gar made sense to me in their role as undercover Forsaken agents - not power players as far as Nae'blis goes). Furthermore, crazy Moridin occasionally forgets that he isn't the DO himself! I can't imagine the DO is fond of that!
What about a results-oriented approach? Perhaps the DO has a longer-term plan. Let's take a look at what Moridin was able to accomplish in his first five books serving in the role of Nae'blis:
aCoS: Made Moghedien his creepy soul-slave. Saved Rand's life during the Shadar Logoth fight, while also apparently mind-merging with him somehow.
tPoD: Told the Forsaken not to murder Rand. Made Cyndane/Lanfear his creepy soul-slave.
WH: Sent the rest of the Forsaken to stop Rand from cleansing Saidin. Failed at that.
CoT: Footage not found
KoD: Told the rest of the Forsaken again not to murder Rand, but to try to murder either Mat or Perrin. Also failed at that. Badly.
So, what gives? Why build up so much Nae'blis plotting and Forsaken infighting for Shaidar Haran to just drop by tel'aran'rhiod and go, "hey, sorry, Moridin's Nae'blis, which was sort of the plan from when we started, so... ta ta". Might it be to foment further plotting? I doubt it. Might it be because of Ishamael / Moridin's logic behind turning to the shadow? We know that the rest of the Forsaken joined up either on account of selfish reasons or because they got mad at the Light (what the heck Demandred/Lanfear); Moridin/Ishy turned because of his interpretation of how the cyclical nature of life and the wheel would eventually guarantee a loss by the Light and a victory by the Dark. Maybe it was this pure loyalty that drew the DO to choose Ishy/Moridin to be Nae'blis?
PS I am aware that this probably will be answered in later books; I just wanted to toss these thoughts out there and see what others thought.
PPS y'all if Mazrim Taim ends up having been Demandred this whole time I am going to flip my lid. There are so many reasons Taim shouldn't be Demandred, however BS's description in the prologue of tGS as being a dark-haied guy with an ultra-hooked nose is worrying to me. Also the fact that Taim closed out KoD's epilogue saying "let the lord of chaos rule", a phrase that had been coined in LoC by DEMANDRED...
PPPS Remember Be'lal? I don't.