Two things here - Loki's blades and strength are not normal and are way the hell past Quicksilver. Thor takes hits from the Hulk and Kurse without exploding, and shots that disintegrate magical Odin-threatening Frost Giant kings without a mark and such. Thor ignores most of what Iron Man dishes, and certainly never got hurt by the guy, meanwhile, taking his armor apart. Loki's blades thus have to be considered special.
And what happens when a NON charged blast from Mjolnir hits Cap's shield? Oh, that's right, all of the old-growth trees in an arena-sized area are flattened, Iron Man is downed and stunned by the blast, and Cap survives because he was behind the shield. So blocking with this shield doesn't seem like a better situation.