Personally I think it fits with the Galactic Empire being both a pulpy science-fantasy villain ala Ming the Merciless and being a Nazi Germany analogue that Stormtrooper armor is as crappy as it appears in the films. The Empire, being cartoonish evil, doesn't care about the individual lives of its soldiers. They're spear carriers meant to enforce the evil overlord's plan but at the end of the day are utterly expendable.
While being an SS stand in, I kind of see Stormtroopers as more concerned with ideological purity, and possibly racial/species purity, than actual combat effectiveness. Being willing to burn to a cinder a couple of moisture farmers without question or hesitation being far more valuable traits than if you can hit effectively as a rogue Princess, farmboy or smuggler.
With the armor primary purpose not being protection but psychological. Making the Stormtroopers appear more intimidating to civilians while breaking down a ST's individuality and sense of self making him more likely to follow the group's orders, no matter how vile.