Like Roger... I have never had sharps come through the soles of my shoes... including on sharp lava beds in Hawaii. Of course, I haven't hiked in Australia... so Rogers observation carries a lot more weight.
I suppose there are some shoes that I could imagine something penetrating through the sole. Ultra thin soled like the vibram fivefingers. More likely though would be bruising rather than a wound since a sharp points force wouldn't be spread out. I certainly notices some short lived pain wearing fivefingers when walking over some fairly pointy granite... but the soles held up and no bruises or punctures. This might have been aided by me shifting weight to the other foot when something hurt.
The advantage of boots (especially full leather) is the protection to the upper foot. The life of more than one pair of trailrunners ended early (before I wore the lugs off the soles) because the uppers were cut up too much to function properly.
One of the things I love (and hate) about my flyrocs is how open the mesh is. Great for ventilation... but the only protection for the upper feet is the added agility the shoes affords me. Even grass like plant material can go right through the mesh and into the foot if you hit them just wrong. When I am going cross country it is fairly common that my breaks involve extracting plant material from the shoes mesh. I find this very annoying... sometimes a little bit painful... but well worth it in my environments for the added agility, good ground feel, and low weight.