Petit Verdot is a bordeaux varietal typically blended to round out the suttle nuances associated with the others. I've heard of winemakers adding it specifically to enhance the color of the wine
Much like Malbec, Petit Verdot is gaining in popularity in S. America, due to the fact that it grows well down there.
I personally believe that bordeaux varietals need one another to truly show their strength as a group. It's kind of like a basketball team here in the U.S. One player typically cannot reap all of the praise for a team success or downfall - the entire group should be praised or reprimanded for the actions.