I really think it depends on the player. In my case, I switched to MP33's shortly shelter I broke 90 for the first time. I was practicing a lot, playing at least twice a week and had pretty good club head speed. Once I got the blades, I started improving rapidly. Still, over the years, I was such a club ho that I kept trying more forgiving sets, then watching as my game tanked. I would switch back to blades and get mu game back pretty quickly. Anything with a big sole and offset was like poison to me. That made me skeptical, and I decided that forgiving irons were a myth. The only benefits I could possibly see were in getting long irons in the air. Dispersion was still terrible in the short irons. A ten-handicapper playing blades isn't going to go buy a set of Ping G-20's and shoot par, or even 7-over. He might pick up a stroke, but real gains will be made by simply playing smarter. Don't try to hit the par-5 green in two and then make an 8. Do punch the ball out of the woods and back into play rather than trying to hit the 5-iron on to the green through a small gap in the trees. Do take 10 minutes and chip before he round, rather than bash a small bucket with the driver.
I think the forged cavities or a progressive set are the best compromises. But I just don't think forgiveness in irons is as important as in the driver or putter.