not exactly. If it blocks high amount of damage from all attacks it will be just another source of resists.
by "block", I mean it will reduce by 50% the hull, heat and energy damages as well as max heat/energy and regen/cooling and resist drain. It is because half of attack is taken by an riot shield, which is a barely more than just plain piece of metal. If hook attack is blocked, it will also nullify the pull effect.
my throughs are riot shield should be able to block a single melee/stomp/hook attack every turn (that is, if enemy executed melee attack and then stomp/other melee at same turn, second attack is not blocked); it can also be used to block other attacks but it will be much less effective (say, 25% instead of 50%) and it will only attemp to block if no attack will be blocked at first turn.
If two shields are equipped, each will only trigger once per attack and so both attacks per turn can and will be blocked; however, ranged attacks still deal a lot of damage + drones are never blocked, and shields have weight and take space of other weapons. I also suggest to make each shield to occupy 2 side weapon slots, so mech with 2 shields will only have 2 top weapons and drone to attack (and having 1 shield is not "must have" due to directly cutting max offensive potential)