Been a while, mind if I pipe in?
If you are using side address capsules like the V series or multipattern MK6 or figure 8 MK8, you need the SGCMs.
I bought mine over the plastic clips b/c I use them in a MK4V/ORTF Kangol rig. They are a necessity since the 4V capsules I use have to be oriented a particular way facing out. The thumbscrews on the SGCM lock the active head/capsule to prevent them rotating and moving from the original position. Same thing applies when I use the MK8 and MK4V in a MS set up on a bar, those plastic clips will hold everything relatively tight but for that added security of not having the set up rotate due to cable tension (KCY cables are stiff), it is worth spending the extra money on the SGCMs.