I used to take my glasses off for anything but the tamest of rides. I'm pretty short-sighted and I always drive to parks, so my view was always that I couldn't afford to take risks with my glasses.
I did eventually get a glasses strap, but even then I took my glasses off on the really intense stuff. Eventually I decided Nemesis wasn't going to knock an arm off them and kept them on. Since then I've been wearing them with the strap on just about everything except Ripsaw (because of the water jets, though I doubt they'd do any harm anyway) and The Ultimate (for some reason I don't trust it with my glasses).
The strap itself was from a local extreme sports shop. It's made by Julbo, and I think it's designed to keep a snowboarder's goggles or sunglasses on. It's very different to Islander's; instead of fitting over the arms, it's a length of cord with rubbery (probably silicone) loops at the end that go around the end of the arms on the glasses. There's a rubbery tag in the middle that allows the tightness to be adjusted. It's been brilliant and I'd recommend it to anyone with glasses with relatively thin arms.
I now have a new pair of glasses with the same prescription. I much prefer the style to the old pair but they don't fit as securely, so I Ieave the old pair in a case in my rucksack with the strap attached. As soon as I'm in the queue for the first ride of the day I change to the old pair and leave them on (including the strap, though loosened off between rides) until ride close, at which I change back.