Grab a frame with stamped or pressed steel drops and, using one hand, try to squeeze them together. Now, do the same thing with a frame sporting forged drop-outs. My experience has been that the forged drops don't move nearly as much. True, there are other factors that impact this test but my money is on the forged drops, as opposed to the stamped steel ones. And...
But try this test with a wheel installed, and neither one is going to flex much.
Forged dropouts do seem stronger than stamped; I have no empirical data but that's how it appears anyway. One benefit to a racer would be less likelihood of mis-alignment in a crash, which would equate to less time off the bike. Not every racer has a team car behind them with a bunch of replacement bikes, and in the old days, none did.