Before anyone reads this, keep in mind that these are just my opinions and since we are on that topic I really don't like the Raiden character that much. But don't confuse it with me hating the game cuz the MGS series happens to be my favorite game series. So that being said Here are my reasons on why I feel that Raiden was forced into the story for the sake of giving the character, and fans of course, closure on his story:
 
My first reason is, the fact that in the middle east he does not make an appearance even when you first encounter the Gekkos, which means that Snake can easily take care of them, well maybe not that easily but nonetheless take care of them, without Raiden's help.
 
Another reason is, the fact that the story is purely centered around Snake and people that are associated with him such as, Big Boss, Eva, Otacon, etc.
 
People may say that Raiden was there for one important reason which is to take care of Vamp. Which to me is not true, cuz if you think about it Snake is the one that really kills him when he injects the syringe into Vamp's neck basically taking out his immortality after that all snake had to so was shoot him and that's it. But for the sake of having Raiden there they make him come in at the time and even Raiden tells Snake to leave Vamp to him while he takes care of the Gekkos.   It could easily have been the other way around with snake finshing off Vamp and Raiden takinh care of the Gekkos like he's been doing throughout the entire game .
 
Another thing is that they made Raiden so damn emo and depressing. It's like at one point you're enjoying the story and the other one they're trying to make you cry by hearing how Raiden is "the lightning in the rain, the rain transformed". Or how he denies having a family when in fact he does. Like they tried to make him sort of like Snake when in fact he's nowhere close to him or snake's mental state. Especially when he gets the whole detailed story on how he was created not born.