Sure $10-15 doesn't seem like that much but with 70 Million 360s sold to date any additional costs rack up pretty quickly. If you were promised a huge cash bonus for cost savings if you were in charge of the development would you throw in components that might be useful down the line or keep the money to yourself?
That $15 could also be put towards a slightly better cpu/gpu for a better peak performance upgrade. More Ram to have room to breathe is nice for developers but with consoles they can optimize the code better for with specific builds.
My gaming PC has 8GB of RAM. Runs everything perfectly.
I doubt you would notice a difference in any games at the moment if you cut it down to 4 unless you have a high end i7 and dual $250+ video cards on a demanding game.