Realistically, it's not. There are some here who believe any update should be a full and clean install. That's simply their opinion. OTA updates are safe, easy, and rarely cause issues.
Tappin and Talkin from my iPhone 5
Why would you assume that simply updating only files that change as being as good of a way to update an operating system? There is much more likelihood to run into random troubles that way then by installing the entire OS.
Your last statement could also read, "OTA updates are safe, easy, but sometimes cause issues" and still be correct.
My phone is important to me, so if I'm going to do something major like an OS upgrade, I'm going to do it the "right" way.
How many time so you see an App Store app update have a disclaimer about some users reporting crashes or other unwanted behavior after updating an app that are fixed by deleting the app and reinstalling? Is the full iOS immune to these kinds of issues?