What is the difference between the Modular Assault Plate and the regular old Mounting Plate? Is it just the ability to hook up the 15mm adaptor?
Yes, that's the only difference. But if you are using a lightweight 15mm FF like me, then the ability to quick release your camera from one system to the next (tripod to handheld, etc...) with the rails attached is a big plus. I also have the Red Mattebox - which clamps on, so everything always stays together. Super simple.