You won't be money ahead buying a few pieces a month. But, like buying a used car, you may have a down payment and can afford a payment, rather than save it all up and wait. Your choice.
Buying one complete means you get it their way. Building one complete means avoiding the Conservation tax and also learning about how to put it together. If you built it, you know more about them than just off the shelf - you can change it, and even have a handle on diagnosing things because you had to learn about them.
Say it's not tossing the brass out for some reason - even new guns do that. First you check with better ammo, a different magazine, see if the buffer tube vent is plugged, check the gas block alignment, etc. A rack gun buyer just sends it back for warranty work - he paid for it, but also pays for shipping sometimes, too, and he's without in the meantime. The builder? He could have solved the problem and still be shooting.
You don't need expensive tools, and you get exactly what you want, as quickly as time and finances allow - including those little things in life that creep up and cause the other guy to sell his rifle because he doesn't have $100 to get er done.
Prices to build are very good right now, and if the builder keeps away from the AR male enhancement offerings, he can put together a pistol or rifle for under $600. Pay as you go has benefits.
Just watch the shipping charges, too, I bought a set of handguards due today, one site was $28 with $15 shipping, the other $30 with $5 shipping. $10 can get you a lot of AR parts for dinner for two on the McD's $1 menu.