left handedness can often provide a benefit in combat (though we almost definetly see anything of the sort in the game, it'll be equal). It may be true that having the shield on the right exposes your heart, and in addition, it hinders group combat such as forming a shield wall with right handed fighters. However, in one-on-one fighting or dueling, or in small groups, left handed fighting will confuse your opponent, who will be used to fighting mainly right handed fighters. Your attacks will come from awkward angles, forcing your opponent to use untrained muscles, and ultimately making it easier to slip past his defences. Therefore the benifits often outweight the risks.
You can also fight backwards
(not an sarcasm actually- there are backward fighting styles in few martial arts :shrug: )
As a novice fencer i can absolutely vouch for how difficult it is to fence someone who is left handed! Everything is weird. Not so much the angles, but your automatic responses to an outside or inside attack. A normal reposte to score a touch wouldn't do a thing, when your normal inside is always their outside.
Do you use shields in fencing?
Because we are here discussing reversed shielding
And IRL reversed shielding makes little sense
It is more efficient for left handed fighters to take dagger in the right hand instead of shield and use it to parry attacks and counter attack