Haven't used them. They are needed if the brass is expanded too much in the web area which can happen for several reasons including early unlocking of the bolt, over sixed chamber. Most of the time I do not think they are needed.
I use the small base dies when forming 300 Bo from 223. But after 223 and 300 bo are fire formed I just use either the Lee 3 die set or hornady seaters. My reasearch led me to believe the small based dies would cut the life span of your brass down because it moves the brass so much. Unless you have a tight chambered gun.
Plus the plain Saami spec dies have been making ammo for Ars since Stoner invented the black rifle in the 60's with out any problems. Just my .02 that they are kind of a sales drive because a reloader with a 30 year old RCBS die that still works has to buy something new.
Dang it I must be a sucker because I have a some small base dies.
I've been using Lee FL dies for some time now with no problems. I load for 6 AR's and have had 0 malfunctions. I was told if you reload for different rifles that I needed small based dies, but like I said they all operate fine.