Although a few sources note that this baffling lack of common sense causes their adversaries to underestimate them, it still doesn't really make up for employing infantry tactics that have long been redundant. That being said, the Mordians do have a few tricks up their sleeves, like flak and carapace armour actually sown into their rigid uniforms (which makes it both elegant AND protective), as well as having heavy armour companies with ''over a dozen'' [[Baneblade]]s '''each'''. They also have 'veteran' companies who wear the incredibly-more-badass darker uniforms who do actually observe real infantry tactics and perform the scout work.
However, one must note that the fact that they're never taking cover means that they're not spending any time not aiming or not firing, meaning that relative to other armies, they could keep you pinned a whole lot more easily. So while they might die off really quickly, they'll do a hell of a lot before they drop. Since their armor has both flak and ''carapace'' armor woven into it,(not represented currently on tabletop) the Mordian Guardsmen are well protected against concussive force, shrapnel, and even against direct fire. So, not only is it a wall of lasers shooting at you, shooting the wall has about as much effect as spitting on a tank. Besides, they probably don't stand in a row, they probably have the first row kneel and the rest stand(leaving the men kneeling more accurate then the men standing as your lose a third stability point as you get closer to standing). Then they have four rows of lasguns firing in a concentrated barrage at the same target. Then remember how long and deep the gun line is, given how numerous the Guard is. That isn't even accounting for man-portable mortar teams, vehicle-mounted mortars, basilisks, manticores, and the armored companies of over a dozen baneblades each.
That's scary and is the perfect tactic for the Guard because their reserves have reserves. It also happens to be the perfect tactic against Tyranids and Orks. The Tyranids are simply that numerous and the Orks are specifically stated to miss anything they aim at, so bunching up into a single target is the best defense against them. Maybe good against Necrons, too, since cover really won't protect you against their weapons. Firepower will.