They can be very useful, if you know how to use them, and more importantly, your team knows how to as well. They certainly draw fire, which allows your team to flank.
At out last big op, there was a wall of shields moving up against us, all basically linked at the arm. They were almost unstoppable until someone threw a grenade from a castle, right over their line. The line fell appart, and one guy mopped em up with a Thompson lol. I got to watch the whole thing go down, laughing like an idiot.