... And another fallacy from internet gun experts is that "different" or "not milspec" is equivalent to cutting corners...
Milspec is desirable in most AR components. Not always necessary in all applications but certainly desirable. Some people talk about exceeding milspec but what really matters is at least meeting the milspec baseline, not falling below as often happens in off-the-rack, sale-of-the-week guns.
When a rifle comes with a commercial instead of milspec diameter receiver extension that's a red flag telling you that you can expect more cost cutting components. Right off you can expect a standard weight buffer and lighter weight bolt carrier instead of the milspec H buffer and heavier carrier which smooth out recoil and functioning. A small thing in many applications but wise to demand when buying an AR. And what about Bushmaster, DPMS, ArmaLite and others' commercial height front sight and shorty carry handles? People replace the carry handle with a third party rear sight that's almost always milspec height which cause problems in zeroing. So many problems that BM sells a 0.04" taller front sight post to fix the problem.
And what's not to like about military specifications on barrel/bolt steel? How does PSA give it all to you for $800 (including FN barrels) and DPMS/BM/WW and many other charge the same for sub milspec rifles? Do they do that to give you something better than milspec or to increase their profit? I'll let you decide.
One other thing, many of us end up with a few ARs over time. How much fun is it to have to remember that one stock won't fit the other buffer tubes and you can't use this rear sight on that AR? Why not demand any AR you buy complies with military specifications as much as possible and then mix'n'match to your heart's content! I certainly demand that of my ARs and have it.
Also, not that I'd likely ever do it but I'm almost certain my BCM, Daniel Defense and PSA uppers would pin right on a real M4 and rock'n'roll. For semiauto functioning they are overengineered (or if you will, milspec) but is that bad?