I have fairly lightweight glasses, and I chose to get a strap for my glasses when I ride. Mainly, if I take a spill and my glasses fly off my face, I'm pretty blind without them so it's going to be hard to find them. I know from experience from the time I was cleaning up some brush, and a stick flew up and flipped my glasses right off my face. I couldn't find them, and I was afraid to walk around looking for them. Due to my nearsightedness I likely would have stepped on them. Even after my wife got home, she couldn't find them either. I ended up getting a new pair, and eventually did find my old glasses out in the yard. Or rather, my dog did and apparently enjoyed chewing on them a bit.
So yeah, lesson learned. I wear a strap while riding. I normally wear it loose but they could easily be tightened up if I have a problem with my glasses slipping down my nose.