Ah, but then why wouldn't you rather focus your efforts on an exercise that is more beneficial/has greater impact on growth? Instead of doing flies next, why not put a little more effort into your bench and go all out so you don't need to do flies next? If you know you're going to do flies after bench, are you holding back on your bench? Could you go just a little heavier? Could you get one or two more reps out? Could you do a 20 second static hold at the end?
Maybe instead of doing flies, you could do a single, 20 rep set of bench (using a lighter weight of course...say a weight you knew for sure you could get 12 reps with, but get 20 reps instead).
Any of the above would be more beneficial than doing flies.
I agree that most people save energy by not going all out on presses just to do a couple of fly moves. And that's wrong, on a normal day I will do only presses and kill it with a 20 rep set like you said. Usually after this I'm done, but there are some days I feel I can do a little more so I hit the flyes...
Nevertheless I think flyes do have their permanent place on a bodybuilding routine, specially juiced and advanced builders, USUAlLY IN BETWEEN PRESSES, while you try to give a rest to the triceps but still want to work the pecs...
I also find them a valuable addiction to the trainees who have hard time trying to take away from the tricep and shoulder recruitment to focus on the chest a little more...