To all you people who keep saying "but it's not concrete & asphalt": have you ever actually run barefoot, or in really minimalist shoes?
Asphalt and concrete are actually some of the EASIEST surfaces to run on barefoot. No, it's not a manicured golf course, but it's smooth and safe without much to trip on, hazards like glass, nails and thorns are relatively easy to see and it allows you to get a good, consistent foot strike.
Running on rocky, rooty trails, or across fields of tall grass with sticks, thorns, holes etc, isn't the walk in the park that you seem to think it is. And something tells me that the savannah or woods that our ancestors ran on was probably a lot tougher on the feet than the local sidewalk/bike path (discounting broken beer bottles and crack pipes, anyhow).
In any event, most of the sharp-object problems are solved by wearing a minimalist pair of shoes. Just like many of our ancestors did, to protect their feet from the pointy rocks.