The origin story of The Boss, Big Boss, Eva, Ocelot, Zero, the Philosopher's Legacy, Metal Gear and the Patriots. How is that unnecessary? Especially if you want to know what the hell is going on in MGS4.
Also MGS2 threw the story completely out of the window, this was fixed by MGS3. In addition Portable Ops and Peace Walker expand upon MGS3.
I hope you've played MGS3 Subsistence, the camera issues of Snake Eater are supposed to be fixed here.
MGS4 > MGS3 > MGS1 > PW > MGS2 > PO