I think alot of people don't know much about what sight radius is or why it matters. Here's a picture to help illustrate it.
The sight radius is the distance between the front and rear sights. The picture above shows 3 16" ARs with 3 different sight radius. Carbine, mid, and disapator. As sight radius decreases, the error in sight alignment is magnified, resulting in a bigger miss.
Again, no issue at all with a scoped rifle, but the sight radius alone makes a full length AR using open sights about 10% more accurate.
If you look at the illustration below you can see how the miss due to the same sight misalignment grows as the sight radius gets smaller.