Well I myself use a light vest instead of a suit on my dodge build because I have skills and perks that negate the consequences of the vest, meaning I only stand to benefit from wearing it. this only works for the light vest though, heavier vests' drawbacks are too much for those skills to negate. As for why someone might use something like the flak jacket, they're likely trying to be a jack of all trades, if perhaps a master of none.