I think that it is a little misleading to state that pros prefer slide film. It depends on what is being shot.(posted 8405 days ago)Probably one of the largest group of pros is wedding and portrait photographers. Nearly all of these shoot neg film, which is designed to be printed using standard photo means. OTOH, photographers shooting for print purposes tend to use slide film. This allows two things: the printer has a color reference to go from, and the editor can look at the film to determine which images to use. Likewise, stock photographers tend to shoot slide film for the editing purposes. (tradition also plays a part here).
For the fine art photographer, slide film can give a greater color saturation. For many subjects, there is nothing that can beat an ILFOCHROME print, which requires a transparency.
David Carper