Back in December, I participated in a friend's nidan randori wherein I got smacked pretty hard in the left eye. (As he said, the sound of his fist hitting me in the video is "pretty alarming.) For the rest of the day, my vision was impaired with everything appearing "lighter" (ie brightness control turned up). My vision was impaired by being a bit blurry for a few days following, too. I wear hard contact lenses, too.
I dropped by an opthalmologist the next day and had the eye checked out. He said that although it looked like there was just swelling of the iris (irisitis?), he wanted to make sure that there wouldn't be any retinal problems (torn or detached retina) so he routed me to a retinal specialist. She took a look at it, dilated the pupil, poked around in my eye for a while, shined some really bright lights into it, and proclaimed it OK. I have a follow up appointment in February to make sure that the retina is doing OK.
Am I now wearing eye protection? No, and I'm not planning on doing so, either. I think such things depend on your own perspective in your training. If you feel like you would be better off with one, then you might want to wear one. As for me, the chance, I figure, of getting hit in the eye (from my own experience) is pretty minimal...