They can definately help,... but not all the time. During late season, I think the toms get decoy shy. The ones that are left have seen a lot of setups and will be sceptical. Also late season has thicker vegetation, and often the birds are in range before they can see anything anyway. Sometimes even earlier in the season, a tom will hang up if he sees a "hen" a ways off. He knows she can see him and will stop and strut expecting her to come to him.
Decoys are just one more option to be used when the situation calls for it.I still usually carry at least a hen, but some days she stays in the game pouch.