A typical question barefoot runners get all the time.
My answer always is "Aren't you afraid of cutting your hand when using a knife ?".
Because the risk of both injuries is about the same and nobody makes a big deal out of a small cut.
The truth is I have never stepped on a big piece of sharp glass that would actually cut my foot. Such obstacles are easily avoidable because you are not running blind and you can seem them very well.
What you do not see are just very small glass slivers you occasionally step on and once in a blue moon they get embedded into your foot.
They do not do any damage and you just dig them out easily after your run. Nothing worse than taking care of a small cut on your finger from slicing an onion.
A little glassy bugger found tonight during inspection of my feet after the run.