Most pros shoot slide film. It's what the industry is set up for, but I think its purely historical in this age of digital prepress. When shooting color, slides give the photographer feedback about the image that can be lost with the print process. Slides have little tolerance for over- or underexposure. You see the result of your choices very directly.
Color negatives capture much more information which is then interpreted through the print process.
In the black and white age, negatives film was standard. It was the complexity of the print process that moved photographers away from negatives to slides. With today's technology, color photo labs with well calibrated color reproduction can be found in many drug stores and small photo shops, turning around print film in an hour. You get the latitude of print film with easy, accurate color. There are two remaining advantages of slides.