This is a matter of opinion, really. The Iron Warriors, Black Legion, and Night Lords are not "generic bad guys" in the eyes of many people. I personally find the Undivided Legions the most interesting and the most flexible from a game play perspective.

They're also going to add in the cult troops later.

Also another thing that the OP seems to forget is that warbands like Death Guard and Emperors children have tended to just join bigger undivided warbands. It wouldn't be an uncommon sight to see khorne beserkers, a sorcerer, and a few plague marines in a Black Legion army. Being chaos is about the spikey bits and how you play, not simply a being in a warband.

im not disappointed at all with there selection they have some of the biggest dogs of chaos rolling with eachother. but I still cant wait to see my blood legion descend on the planet!

A major problem with the Cult Legions imho is that they do not exactly get along with each other as a result of their dedication to but one of the Chaos gods. As such, getting World Eaters and Emperor's Children to fight together might prove difficult, as could getting Deathguards and Thousand Sons to cooperate.

Also, the current legions very nicely fit the generall class system:
Heavy Infantry/Iconic Char: Black Legion
Assault Infantry: Night Lords, who literally invented Chaos Raptors
Fire Support: Iron Warriors, who are all about Big Guns and Mechanics

S'not about who I fight with, s'bout the colour of my armour the enemy sees when i'm charging them down.
Blood red soaked in red blood.

I love them having Night Lords and Iron Warriors and am willing to chain axe to the face any who disagree.

I seriously hate the idea of pigeonholing legions into classes. We don't need people min-maxing their legion choices and specialties just for higher stats, yelling at others for not doing the same. I got enough of that from WoW.

-ok if this is true its all good then, I mean we dont need all of them, but the more the merrier

it would have gone 2 ways:
1-one legion for each chaos god and one chaos undivided
2-five chaos undivided
now the former sounds more fun but think about it, each god having their own specialty and playstyle would cause tons of balance issues, there would always be the one stronger than the rest and the one that sucks. this would cause people to pick the chapter that plays better and not the ones they would want to play.
with 5 chaos undivided you can choose the faction you want without serious repercussions and worship the god that benefits you most.
additionally with the former choice you would be limited to one god.
with chaos undivided you can mix and match the god(s) of your choice for far more adaptability and customization.

If you are worried about Legion thematic, as of certain specialists from different Legions, then try to perceive it another way. Chaos God dedication influences your choice of class(Khorne-melee/charger, Slaanesh-sniper/assassin, Nurgle-ranged/apothecary, Tzeentch-caster/summoner, Chaos Undivided-tactical/buffer), and according to this you can have custom deity-related heraldry. I have mentioned that before, that can be applicable to the other races.