Honestly I think the joint force is most interested in our combat support because of capacity shortfalls in high demand/low density fields. The "every Marine a rifleman" ethos produces a unique kind of support Marine who is motivated, disciplined, and expeditionary.
I love the infantry but we can only do so much without more armor and mechanized vehicles in a high intensity conflict. I have argued before that the Marine Corps should drop our tank battalions and form partnerships with Army units similar to the Royal Marine model. The British Army has attached armor and cavalry units to the Royal Marines.
I've always thought it was speed and versatility. A MAGTF can be moving with a wide-array of capabilities in a very short time. It's interesting though because the GCCs most frequently have the say on what capability they want - so aren't necessarily looking at it the same way as the joint staff.