You surely don't have to use TSS to kill a turkey, but there's no doubt that it's the best shot material and most folks who turkey hunt want to use the best. Someone said it's like fishing with a Zebco 33 - there have been millions of fish caught with a Zebco 33, but you will never see one in use at high level bass tournament. They are functional, but far from the best. Someone could host a tournament with a requirement that you could only use a Zebco, and if the money was good enough the pros would fish it. And they would catch a whole lot of fish, but not quite as many as if they could use regular tackle. It would depend on the lake, but they would likely catch something like 95% of what they would have caught. That 5% is often the difference in winning or not.

You can probably kill 95% of the turkeys with lead shot, but if you hunt long enough the TSS will be the difference in killing one or wounding one.