The effectiveness of smith machines depends on the training goal.  If it is a fitness-based goal such as developing muscle hypertrophy (an increase in the size of muscle fibers) for pure aesthetic reasons then the smith machine allows the ability to focus on a specific muscle during a limited range-of-motion, so it can be effective.  However, a traditional smith machine does limit movement to one plane of motion, which is the sagittal plane, so it does not provide an effective training overload for people training for performance-related goals, such as competitive athletes who require the ability to produce, reduce and stabilize movement in all three planes. 

To enhance athletic skill, a training load should challenge the body to maintain stability in all three planes as well as accommodate for the dynamic forces of gravity and ground reaction.  Instead of the smith machine, people who are training for improved sports skill or enhance sports performance should select equipment like medicine balls, resistance tubing, barbells, dumbbells, kettlebells and multi-directional cable column machines which are the most effective for recreating the forces an athlete might experience during their specific sport or competition.  There is one case in which a smith machine can be an incredibly effective tool for athletes training for performance enhancement—as a place to attach rubber resistance tubing.