I find benefit from the c-stocks ability to shorten the gun for storage, the M-4 is not a small gun and would not fit in the case I use without it. I am eagerly awaiting the rx-arms 6 position recoil tube. I can shoot mine presently from either the middle or third notch but neither feel quite right. I'm confident the extra positions will rectify that.
I have mentioned this in the instructions on my website, but wanted to make a note here. The Rx 6 position tube should be installed leaving around 1.5 threads exposed in the reciever. In my experience this is how OEM looks before it is removed. And my tube is designed for this.
I bought mine because I liked the option to change the length. I am a big guy and the longest setting and middle setting work great for me. It saves a little weight I am sure too.
Lets all be honest to - a lot of the reason everyone gets it is because exactly like you said: it is how a Benelli M4 SHOULD look and it just looks cool.
For me it's all about the flexibility.
The half way position is PERFECT for a red dot on a low quick release mount.
The fully opened position is PERFECT for the factory sights.
It basically gives the user the ability to utilize the gun in a way that fits their own personal shooting style.