I'm siding with Nahkuri on this, I don't see any valid reason to have an asymmetrical swing area that favors RtL attacks and makes LtR attacks essentially more difficult (even if skilled players can perform a LtR swing flawlessly and with precision, it still requires additional effort). Why this system is in place currently is probably because the devs want RtL as a sort of "default" attack into which you don't need any modifiers - and, granted, it's more natural for a right-handed person to perform an attack swing from the right side.
I wouldn't think a 50/50 division right across the center of where your character is facing would make the combat more random; it would only mean that if you want to perform a RtL swing you move your cursor slightly to the right before clicking, and for LtR you move your cursor left an equal amount. Nothing difficult or ambiguous about that, you can still easily do RtL attacks as much as you like.
In my opinion the best solution would be to simply map LtR to a different button than RtL. Left and right mouse buttons would be perfect for this. Since cursor movement is primarily used to rotate your character, it's less than ideal to use that as an attack mechanic as well.