Internal gear hubs do have slightly more drag than a well maintained derailleur system. This means that if two riders are equally strong, the rider on a derailleur equipped bike will have a slight advantage. IGHs are also generally heavier than a high end derailleur setup, even taking into account the removal of derailleurs, shorter chain, one shifter, etc.

Also, the primary advantage of IGH systems (in my opinion) is increased reliability and decreased maintenance. Pro cyclists generally have a mechanic and perfectly maintained bikes, so this advnatage is less. In fact, if there is a problem with the rear wheel (a flat, for instance) it is actually more difficult to fix than the same problem on a derailleur bike.

My main bike has an Alfine IGh and I love almost never having to work on it. I dread getting a flat tire, though.