Whilst there may some truth in the pec deck being mainly the choice of inexperienced gym-goers who shy away from entering the free weight section of the gym, we must not disregard the exercise totally in favour of free weight alternatives (such as the dumbbell fly).
The pec deck isolates the chest muscle in much the same way the dumbbell fly does, with the only other assistance coming from the front deltoid (shoulder) head. The negative aspect of the pec deck is the fixed range of motion, which may cause some people shoulder discomfort, which is especially unforgiving if not performed correctly. That said, some athletes like the pec deck due to the greater perceived focus they can place on chest recruitment, due to the support given to the lower arms, compared to that of the dumbbell fly.
The pec deck is also a popular choice among bodybuilders who wish to pre-exhaust their chest at the start of a chest session, or wish to perform drop sets (this is an advanced weight training technique where a given resistance is used for a number of repetitions which takes the bodybuilder close to muscle failure, and then the weight is quickly reduced allowing more repetitions to be performed, and so on).