Part 4: Raiden is Cyborg Ninja
PART 5: Beauty & Beast Unit
Obvious Reasons
Every Metal Gear game needs to have a handful of bosses scattered throughout, providing a special challenge and helping to move the story along, and MGS4 is no different in that regard: the four sexy ladies each get their moment in the spotlight, fighting Snake in unusual ways and inevitably falling at his hands. So what are the obvious reasons for their design?
The biggest reason is undoubtedly the 'psychological warfare' aspect that Kojima emphasized from the beginning. The whole idea of "Beauty & Beast" is to juxtapose the pleasing with the horrible, and this is a perfect example. Insane, tragic, deadly and sexy supermodels with a uniquely annoying emotion — it's quite obvious that we're supposed to be uncomfortable with fighting them. To reinforce this theme, we are forced to fight each of them without their "beast" suits after the battle, when their fragile (and oh-so-supple) self is exposed. They don't attack us, they just want to get close and hug. It's messed up, all right. Who can really feel great about shooting some pretty lady who's simply walking towards us with her arms out? We don't know whether to be happy or sad when we smack them around.
Beyond that, the names are obviously ripped off from previous enemies in the series. Laughing, Crying, Raging, and Screaming are parallel to The Joy, The Sorrow, The Fury and The Pain from the Cobra Unit, while Octopus, Wolf, Raven and Mantis are stolen from FOXHOUND. Why? Because it's the last game in the series, and Kojima is trying to ressurrect memories of the old games. Nostalgia/tribute is a huge theme in MGS4, and one could even argue that it's part of the 'psychological warfare' as well.
A third reason is to show off graphical prowess. Kojima made a big deal about the face-scanning technology used to capture the likeness of the four models used for the game, and the only true way to demonstrate this is to take the image of real-life people so that it can be easily compared.
The Hidden Reasons
I would argue that the real psychological angle to the B&B unit is their uninteresting and superficial design formula, which robs the players of motivation to meet or care about them in the first place. It's Kojima's way of saying "I don't care anymore". Unlike the charming, intriguing characters of the FOXHOUND unit, or the bizarre drama of the Sons of Liberty unit, these women are one-dimensional ripoffs of much, much better enemies. The Cobra Unit lacked 
personality as well, but it fit with the themes of carrying an emotion into war and was compensated for with awesome designs. I'm sure their theme was thanks to laziness on Kojima's behalf as well, but the B&B unit takes the cake for being half-assed.
And it should be no surprise that the bosses are uninspired and formulaic, since that's how the whole story is. Rather than introducing a meaningful team of diverse specialists from the corners of the globe, each with their own history and agenda, we're given a squad of utterly dull puppets, blindly carrying out objectives with one emotion, each with forgettable sob-stories told to us by Drebin. The way it's set up, it's like they're trying to make us skip their explanation. It's this bad (scroll down to #2 on the list and try it yourself).
In the end the B&B unit helps to illustrate how much Kojima didn't give a shit about the story, while knowing how to appease the mindless mob of fans who were demanding a sequel. Constantly showing erotic angles of the characters while their out of their suits and allowing you to take pictures of them and make them dance in a special dream room? I'm sure it serves many important purposes, but ultimately it serves to demean the integrity of the characters and distract from the severely lacking creativity of Kojima and team.
PART SIX: Liquid Ocelot is Silly
