I've never hunted turkeys and I know nothing about turkey hunting.
Please tell me why you need such a huge load for turkeys?
I thought that Turkey hunters did a lot of sitting and waiting for the bird to come to them, so why do you need such a light gun?
Aren't turkeys usually shot at close range?
Like I said I know nothing about turkey hunting.
All good questions, and Joe has answered it very well. You can certainly get by with a lighter load; the heavy load just allows me to have a wider pattern that is still dense enough to kill the turkey out to 50 yds without any concern. I shoot a 2 oz load in the 12 gauge; some people shoot 2.25 oz and a few even heavier. Gil is hammering them with a 13/16 load in his .410. Using the heavy load allows me to still have 12 gauge performance in a light 20 gauge gun. There are some folks using a 1 and 7/8 oz recipe, but I have settled on the 1.75 with either 8s or 9s.
I don't do a lot of sitting in turkey hunting in the southeast. I hunt all day, and its not uncommon to walk 5 miles or more in a day's hunt. And if I get on a turkey, I have to keep the gun pointed in his direction while calling to him - you have to be ready to shoot when he appears. It is much nicer doing all this with a 5 lb gun than an 8 lb gun.
As far as range, I've shot turkeys this spring at 38, 30, 25, 45, 38, 25, 27, and 41 yards. Some people want them to get closer, but I had rather go ahead and shoot him at the first available shot that I am 100% sure I can make.
I hope that explains it a little better.
Thanks to all for the replies.
...by the internet stalker. 
In all fairness to jOe, those six-month old baby hens he shoots in the fall are a lot smaller than a mature gobbler. He might need all the magnification he can get with those birds. Steve, good point about those Nitros. I recall one hunter who dissected a Nitro 4x5x7 load and found a high percentage of the shot in the shell was Pixie Dust 9's. (those 9's were 12 g/cc compared with our 18 g/cc 9s.) People who hunt turkeys from glass porches shouldn't shoot 9's out of an SBE with a Swarovski scope and criticize others who shoot tungsten 9's with red dots on their guns.