This scan is somewhat inappropriate as it involves his other shield which can expand. But it demonstrates a point. The convex (outward curve) curvature of his shield helps to deflect away a direct stream of force. In other words, a stream of water from a firehouse splashes outward when hitting the shield. Although the shield can't protect his standing body, if Cap can shrink down his stature or vulnerable points, like he does here by going into a perpendicular stance with respect to his shield, he could survive a full force blast and at worst, just be knocked away. Of course, a lot of us patriots think he wouldn't get knocked away cause he could take shots from Hulk and not get knocked away... but anyway, that's the worst that could happen. But do you see how something that you think would engulf Cap, doesn't? Again, them beams of Cyclops aren't going to curve around and hit Cap from behind once being deflected. Unless now he has somehow gained the mutant ability to alter the stream's path midflight...