It does help nesting success greatly, for waterfowl and upland birds. Delta Waterfowl did a study on nesting success and how predators affect it. I can't remember the exact percentages but it was stunning! Somewhere in the ballpark of a 70-80% increase.
You're right that one man can't get rid of all the predators but trapping can reduce them enough to see dramatic increases in nested hens and number of chicks. I began trapping my nest areas this year after seeing how much success a friend had on his private lake. HUGE DIFFERENCE.
If you're a bird hunter you should either learn to trap or have your property trapped by a responsible trapper. It pays off in one season.