Gathering the info I could collect with my observation so far in One Piece anime:
Most of the famous swordsmen have injuries on their eyes.
Ex: Admiral Ishou, Dark King Silver Rayleigh,
Perhaps It looks like it's considered as an ability of Swordsman to sense the things/people around. Zoro always thrived to learn this technique and succeeded too.
On the otherhand it looks like it's (Kenbunshoku Haki) considered an ultimate ability of swordsman to fight without using Sight sense. So losing sight might be A KEY to AWAKEN ultimate Swordsman powers.
Zoro might have intentionally cut his left eye to awake this ability.
Or he might be still training with his left eye closed. Might open when time comes.
But like other OP fans, I wish to see something special out this.