I had the exact same discussion with someone a few weeks ago, so I'll just copy and paste most of what I said then:
What's the first thing you do when you get ready for a set of flyes? You head over for the lighter dumbbells.
Flyes are a huge step down because they are an isolation movement. They remove the triceps and shoulders out of the exercise, which means you will seriously drop the weight down. You're expending a ton of energy for a little bit of overload.
Isolation = reduced overload, and less overload = less muscle fiber stimulation, and of course less muscle fiber stimulation = less growth. It really is that simple.
A lot people do flyes because they think they are "sculpting" the muscle, but hopefully by now some people understand that you cannot change the genetic shape of your muscles. You can make them bigger, but that's it. So what do you think will make the chest bigger? Pressing 315 pounds or doing flyes with 70-pound dumbbells?