The following paper, written over ten years ago sheds light on the USMC thinking. Its a Masters Degree Thesis re the 53E, but addresses the V-22 vs 53E roles and missions concepts:
http://www.dtic.mil/cgi-bin/GetTRDoc...92&Location=U2...
One Approver, Col Paul Crosetiere, was the 53 Class Desk Officer around that time and one very, very intelligent Marine Aviator.
It also might help readers to know that the USMC had underwritten a CH-53D 50,000 lb flight test qualification program in the late 70's ( which was later used by the IAF, but not the USMC ), so the USMC is/was well aware that the V-22 could do some but not by any means all, of the 53D missions.
Someone will inevitably raise the question of whether moving artillery is a USMC medium lift mission, but it appears that they have decided ( ? ) to make it exclusively a heavy lift mission.
Thanks,
John Dixson