Yup. For me too ... a peace of mind thing. One could argue that you don't even need fitted clubs at all if you're even remotely "normal" sized. My old off-the-rack irons have been used only a couple of times since I got my new, fully fitted ones, and I saw very little difference in score. (One of the times was one of my best scores ever)
But since I knew I was buying clubs for the first time in 15 years, and probably won't be buying irons for another 10 or more, I figured why not upgrade to the fancy shafts, and the silly "puring."
Like you, I now know that any issues are not from the clubs, but only my swing.
That is true, but I do see a dispersion difference between iron. In my fitting, when I went to a less stiff flex, I would start missing them right and left. With a stiffer flex I was hitting the ball in a much tighter area. Basically I wasn't trying to adjust on each swing on how the club loaded. It wasn't that bad, if you imagine a center line down the fairway, I went from missing 5 yards left and right, to must missing on the left side. In the end it is a comfort knowing I don't have to worry about the club shafts.
This happened with my Ping i25 versus SLDR driver as well. The Ping golf shaft was way too flexible for me. I would be missing it both left and right. The SLDR was only missing left, and it was a consistent ball flight.
Lucky for me, all these shafts were basically just free custom changes. That is one criteria I try to go with. I can play any iron if it looks good, so I have a look criteria (I know, it makes me a shallow person :p), but I also look at what options I have for standard shafts. Why I tend to go with Mizuno and Titleist over Callaway because they offer more options in better shafts.