really. some say flies is a worthless exercise
Not at all. Some people are great responders to pressing exercises, thus all variations of dumbell/barbell presses give them big pecs. I'm one of those people who responds to squeezing exercises far more then pressing exercises, thus heavy flies pack on the mass for me. It was a tip I got from a musclemag article by gary strydom a long time ago and it paid off. All benchpress got me was bigger triceps and front delts.