Gambling was somewhat of a staple game mechanic in Diablo II in order to balance out the infinite flow of gold from monsters and selling equipment. Whether it will return in Diablo III or not is unknown at this point.
Why Have Gambling
Besides possibly being a fun way to spend your money, Diablo III will have less need of a "gold sink" than Diablo II had. Instead of unlimited amount of income, and no expenses, the gold economy in D3 will be more balanced, according to the D3 Team. The practical need for a gambling vendor is thus gone.
Why Not Have Gambling
The drawback with having a system where anyone can potentially get really good equipment from using gold alone is a stronger emergence of gold farmers, and gold selling on the Diablo III realms. Just having this potion will likely make the market for selling gold for real world money a lot stronger, even more so that would come from having a balanced market in the first place.
A lesser drawback could be for new players who do not understand what it's about and spend hard earned cash on a lowly cap.