Not a single one of my current ARs comes as a complete package.
I could probably be just as good (or bad :alcoholic:) with one of the basic out of the box packages, but I find that over a few iterations I start to build preferences and opinons, and those generally turn into progress of some kind.
I try to be cost effective and maintain a favorable cost:benefit ratio. Sometimes this means range time and driven practice, and sometimes it means spending a bit more on a barrel or buttstock. I want to have as "perfect" a weapon as possible without being afraid to shoot it and build a twin of it, just in case.
I also have more AR lowers than I have the ability to carry if they are complete, and fairly few people have won gunfights with a stripped lower. So I tend to buy upper receiver groups and mate them to the lower that best fits the application of the upper.