If you look at the vanguard class emblem, it's almost like it's missing something.
It has the titan, warlock, and hunter class emblems on them and then a 4th, almost just...space wasted slot.
Is Bungie just being lazy? Probably.
Perhaps it's just insurance in case the team decides to throw in a 4th class.
I like to think maybe they got something random in the back of their minds.
Food for thought...
The hunter, the titan, the warlock, and the duality between darkness and light where they stand.
The darkness isn't just the shadow tempting us with stupid frosty the snowman powers, we serve the light by walking in darkness. We fight. We kill. We go into dark places and leave them darker for the blood of our enemies spilled there. We serve the light, but we walk a path of darkness.
The stripes symbolize the light and the dark that is our nature.
I would argue that the best image to show guardians nature would be to show them walking a line between light and dark. The bright side casting a shadow of light into the darkness, while in equal parts the dark casting a shadow of darkness into the light. That is the place we stand.