Generally speaking, if you're inclined to build one, do it. If nothing else it will take the mystery out of the rifle.

Warranty on a $500 modular rifle? An AR is an AR. It's a modular rifle that anything can be replaced from a zillion vendors cheap enough. There is really very little cost exposure. Heck, you'll go through $500 of ammo quick enough. Warranty makes sense for those who cannot or don't want to understand the rifle. And that's cool... lots of stuff I don't feel like fooling with. But since you're willing to tackle the build to begin with, a Sport warranty would seem to have limited value.