Trust me on this one. Most vests weigh at least 5 pounds if not more. They do not breathe and they generate the same heat as a winter jacket. Even if you are in great shape and well conditioned thats still 5 lbs to slow you down.
Plates weigh like bowling balls. One ball on the back and one on the front. They have actually been around forever and the military didnt use them until the last several years. They used them mainly because of the politics and media not because they felt it was a good idea.
Study after study for the last 200 years say the same thing. Carry around more then 35 lbs and you become less effective as a foot soldier. That hasnt changed. Wearing 40 lbs of armor on top of your usual load is suicide. Would it make sense to carry two bowling balls through a battlefield? I guess it makes sense to those who believe two heavy plates covering up a small portion of your body will make you a better soldier or Marine.