The idea behind limiting character customisation is essentially the same like not delivering female player avatars: The Art-Style.
With BRINK's - so called - hyperrealistic art-style SplashDamage want to make sure, that every item inside the gaming-world blends into this art-style very well.
That means an APB-esque customisation is not possible. And at the same time a bazzillion items would not work either, because they'd all have to be very, very similar looking.
Yes, that's the reason they do not want to include "sliders" for the face---so that the characters can fit the artstyle of the world.
(BUT that's not the reason why they did not include females. It's a resources and balancing issue-thing, NOT an "art-style" thing. -_- )
I think they said that the archetypes are there to give an idea of the customization---and that the archetypes only have about 75% of the stuff available. There will be more to be unlocked, and, seeing the gallery, I think there's a good variety of mix-and-match items to give your character a unique look. ^_^
There's only so much customization you can do. No game is perfect in that aspect (even games that include "sliders" and PC modding tools like the Sims, Dragon Age and Elder Scrolls games,etc).