Fair enough about the redguards thing, I think I'll drop that since it's not what I wanted this thread to be about.
However I would still like to point out that the Fluff behind the gameworld is ultimately there to make the game better. From a gameplay perspective does it doesn't really make any sense to have 3 different races (Nords, Redguard and Orcs) that cover the same basic archetype (Fighter)
Choice I think is the key. Having the choice to play as these established races. They also don't really cover the same archtypes as they all fall into many sub groups. Also from a roleplaying perspective is the choice of culture.
Nords are strong hardy warriors who excel in two-handed weapons and are also good with sword and shield. You imagine Nords in ragged armor, maybe even light furs, and still fighting with the strength of someone in heavy plate.
Orcs are a brutish nearly beast race who rely on onslaughts of attacks and deep resilience to damage. They have a blood rage, most likely a trait earned through genes within the race, that allows them to fight with a sudden burst of strength.
Redguards fight often with sword and shield but also duel wield. They fight with finesse and quick planned strikes rather than their brutish counterparts.
You could simply think of Imperials as a "basic" fighter. Think of them as a Roman soldier obviously. They fight in a very taught fashion.