Mechanically it would be heavy and cumbersome, but could absorb more damage than whatever grade of armor it would be (ie Protek armor can take up to x caliber of damage dealing, Protek Riot Shield could take up to y caliber of damage). Maybe it can be utilized as a combat weapon through whatever skill, but I think it would be an interesting use of a defensive item in order to take more fire or handle larger groups than typically able to. It would be used one handed, and so could have an off hand weapon, but I suggest that using the shield would stop you from having counter attacks you would normally have with a weapon to help balance it out.
Maybe it would be widely available (like IRL), or maybe they would be niche items. I just think they would be a nice item to help make more defensive positions and roles function in a different way.