Just remember to respect copyright laws. If you are creating an ad campaign, for instance, and expect to use a photo many times, a truly royalty-free image is probably a better deal. Any time you are on a tight deadline, using stock photos makes a lot of sense. Even trying to set up a shot yourself with a digital camera can take far more time then searching the Internet and downloading a usable stock image. Hiring a photographer is expensive, and shooting your own photos involves your own time.
Stock photos are almost always the least expensive alternative.
With images from many photographers, stock-photo houses offer plenty of styles, subject variety and flexibility. Well, think about it. No photographers to deal with. No need to run around taking photos on your own. That said, there are reasons to take your own photos or hire a photographer.