I would never say they are "necessary". To use decoys or not is one question that never has a solid yes or no answer. Sometimes they work great and other times they can ruin an opportunity. Its all part of the reason I love to hunt turkeys so much. Each one, each situation, can be different. To use decoys or not is probably the most asked question I ever get.
Here are a few of my "rules" on decoys, and like any rule turkey hunting none of them are a sure thing:
Hunting an open field I will usually put decoys out. I have had the best luck with a couple feeding hens. I don't like the head up "alert" hens.
Hunting thick timber I rarely use them.
I don't use strutting gobbler decoys unless I have tried several other things on the same bird and nothing else is working. I have had strutters scare birds off more than they have worked. However, when they DO work its awesome.
Again to the original question.... I have taken a LOT of birds without them so I never see them as necessary.
Getting long winded, but the high priced decoys I see now that look so real are NOT necessary. I use the inflatable decoys because they are so easy to pack in my vest. Those $100+ decoys are made to lure in hunters, not turkeys. JMO.