Here is what Jeff Willet (IFBB Pro and 2003 Team Universe Overall Champion) says about flyes:
Dumbbell Flyes (incline, decline, flat)
The first thing you need to ask yourself before choosing an exercise is: Will this provide maximum overload?
Flyes are an isolation movement for the chest because they remove the triceps and shoulders out of the movement. However, isolation reduces overload. Less overload means less muscle fiber stimulation.
Another reason people mistakenly do flyes is to shape the muscle. Let me make this perfectly clear, you cannot change the genetic shape of your muscles. You can make them bigger which may appear to change the shape, but you cannot change the genetically predetermined shape of your muscles.
Flyes are basically a worthless exercise in my book. You expend a lot of energy for a little overload.
Cable Cross Over
This falls into the same category as flyes. Worthless. A lot of effort for a little overload. Cable Cross Overs do not build muscle, they do not shape the muscle. All they do is exhaust the muscle.
Pec Deck
This is the same as flyes only they have made a machine to do them on. A waste of time. Too much effort for too little overload. Leave this one to the Saturday morning fitness crowd."