I'm sure it's fine for the average shooter who'll hardly use it. As for the difference, quality of materials and features. We all know that most M4 fans want after-market parts. Might as well do the math and buy a rifle that someone has already set up the way you like or has parts you can take off and trade. This is the whole premise behind brands like Magpul.
Not trying to be snarky here but, it's been my experience that the person behind the trigger determines accuracy. Most of these rifles with a carbine and up barrel will shoot well out to the max effective range if the handler knows the fundamentals of marksmanship.
You missed my post about how hard I have run my daughters DPMS. VERY hard and with absolute reliability. Your point about after market parts is correct, but the base rifles are little different in function and reliability.
Your statement about any of these rifles being capable of doing the job out to max effective range is exactly what I'm saying.
The fact that the less expensive rifles are reliable and accurate means that the ONLY functional difference are the bells and whistles that people want on the rifle. If you do the math, many of the "premium" rifles are charging WAY more than those extras would cost on an inexpensive rifle if you added them after market.