I have also worn glasses since I was a kid.
Horn rim glasses (plastic frames) are bad if you get hit on them. They tend to shatter nicely. In a way thats a good thing because shattering takes energy that would otherwise be applied to your face. If you get hit or hit someone on the corner of the frame it will tend to put pressure on the joint of the glasses and on the bridge of the nose. Frames that have the spring loaded joints are better because they tend not to just snap, but I have had a few of those break as well. Any energy that is not taken up exploding the glasses tends to get transferred to your face somewhere.
The only sports glasses that seem to have any merit at all for martial arts are the really geeky looking black horned rims with a kind of rubber bridge. Sports glasses like the kind that you wear to play racket sports like squash are REALLY BAD. The ones that I am talking about have a solid wide plastic frame and elastic to hold the whole thing on. This type of frame sits on your nose and nowhere else. If you get struck straight on or from just about any angle the force transfers right to the bridge of your nose. I broke my nose in about 20 pieces that way. The glasses made a nice little mold of themselves in my nose. If you want to ever see just how much a nose can bleed...well this is one way to do that.
Incidentally, when that happened I reacted as I often do when something is chewed up on me. Without a lot of forethought I instinctively do something about it. In this case, I felt the compressed bones, saw all the blood, and thought gee I better do something to put all those bones back in place. I held my nostrils together and blew to create back pressure. This re-inflated the crushed area with a nice sound like chewing on a life saver and did in fact save me some time and money later since the nose did not need resetting. Afterwards the doctor stared at me when I told him and he quietly suggested that it was a good way to get infected. Instead its probably better to let them go in and manually push it back out. It bled a lot too.
Sorry for the graphic nature of that but I thought it would be useful to describe what can happen as a result of getting hit on the glasses.
My favorite type of glasses are metal wire rims. If you have a strong prescription (thick glasses) the smaller the lens size the better because larger frames will weigh more and have larger and more sharp edges. Modern steel frames can be made of a variety of alloys and one popular material these days is titanium. Titanium is supposed to be really flexible as far as metals go. I have found that titanium frames are extremely light and comfortable which I really like. They do seem to be pretty flexible compared to my older frames. But they tend to fatigue around stress points and will suddenly just snap in two sometimes without notice. That is if subject your glasses to whomping like I have on occasion in the past. Keep in mind that the kind of stresses I put my glasses under are things like grappling with them and repeatedly smashing them into someone's side or having someone roll over them on your face . Metal frames bend back. Plastic ones don't.
And if you are over 50 as I know some of us are, you can also have the fun of dealing with bifocals. Some people get dizzy or disoriented when first wearing them because your field of vision warps when move your head back and forth. It seems to be one of those thing that you can simply get used to though.
As far as preparing for trouble goes, I think practicing under all possible scenarios with glasses is a good idea. If you don't then you will not be ready for some of the things that might happen. For example, if you are wearing them and you suddenly find them gone you can learn to just deal with it instead of being disoriented (much). If you get hit in the glasses and they have an edge expect to get cut. Even light hits can cut you. Harder hits can lay you wide open and its possible that the lens or a piece of the frame could slip into your eye. I am not sure if wearing glasses makes you better at getting your head out of the way because of all the negative reinforcement you get. More likely it makes your friends more wary of hitting you there and so you get biased practicing happening. Taking shots with gloves would be dumb.
If I loose my glasses my vision of course goes from 20/20 or there abouts to seeing vague blurs where the opponent's head should be. I seem to err on the side of extension.
I noticed that I tended to follow through more rather than less in that situation. quite naturally. But you loose a lot of sensitivity and you have to rely on other things. ARe some ranges better than others when you cannot see so well?
At this point, I use contacts most of the time. Contacts actually give some protection to the eye but they can also be swiped or rubbed and get displaced. Having a contact go missing is also a freaky thing. You can then have the situation where one eye is focused and one isn't. We all have dominant eyes. If the dominant eye is the one that looses the contact its a little more disorienting than the other way around.
Karsk