1) Adjustable stocks do allow shooting squared up - for CQB use. It's a technique that is taught to improve the presentation of body armor protection. It reduces the openness of the off hand arm aperature in the armor. It's been criticized as making yourself a bigger target to let the plate take the hit. I wouldn't know, as I don't plan on doing that. It's definitely not the proper stance for range and target shooting for precision.
2)Quad rails are just one answer to the options - a handguard tube is the other, and quite common. Rails can be added where needed as needed, which can make tubes lighter than quads. Ladders won't be needed to cover the aggressive CNC machining, and the expense is reduced overall. Again, quads are an institutional answer, pretty much a fad in the civilian world where everyone copies the military answer without thinking through whether it's of any value to their use.
3) I don't propose saying slings are inherently evil and should be banned, they have a purpose. I am saying most shooters picking one are doing so because they copy the latest tacticool fad without thinking about their application. It's not wrong to use the carbine without a sling at all until a NEED is identified that a sling will address. In tactical training, the Infantry School had us remove them to promote keeping the weapon at the ready. The obvious disadvantage was that soldiers on patrol get tired, and fatigue would cause some to just sling the weapon on their shoulder - when it was supposed to be at the ready, at all times.
Considering the source of the training and policy were experienced combat vets, I have to ask why their recommendation isn't valid? I'm passing along the same thing I was taught - learn the weapon and it's use, then address needs or weak points when the user knows what they are.
Just popping off saying "This or that is a mandatory tactical item" is pointless without the shooter knowing when it will be necessary or why. If they are a day one user with little to no education or experience, the finesse points of advanced tactical use is premature.
As soldier in an MP unit my last 4 years, I got a good idea of when slings would be helpful - in urban use. Not while IN a vehicle, like a HMMV, or low crawling under a foundation. In those conditions, they can and will be negatives, and have to be controlled. They were also the typical silent type, attached at the swivel on the gas block. That is not the best type, nor the best place. For my unit at the time, they were redundant, as we were never armed with both carbine and pistol. The weapons were considered in two separate and distinct uses, fighting tactical, or street use admin. LEO's don't do that - they can and will have both.
This is where the three gun competition view muddies the water, as transition training from carbine to pistol and use scenarios make having a sling almost mandatory - but that doesn't happen for the majority of armed soldiers and citizens. They usually choose one OR the other.
It boils down to making the mistake of applying advanced tactical techniques as a blanket answer to any situation or use. My opposition to adjustable stocks, quad rails, and slings is because of that mindset. It ain't necessarily so. They have disadvantages a pro has to compensate for, and their use - such as a squared fighting stance - may not even be known to the public.
Advanced accessories and techniques aren't for newbies. They need the basics first, not a "hit your credit limit or you aren't a man" mindset.