Should the DOD allow intra service transfers?

Perhaps. But only for SM's who have completed their first enlistment term and are in the re-up window. If the needs of DoD dictate a cross leveling of personnel, then branch transfers in same MOS, or re-training into top priority MOS's can be authorized to fill that need. The SM would in effect re-enlist for another branch with no break in service.

Yes, but specific training on that service should accompany it to the extent that the gaining service is satisfied that you are properly oriented and integrated into their service.

As it stands, it depends on the separate services to integrate other services personnel in their own way. The DOD would have to come up with one for all the services to abide by. As it stands smaller services such as the Marines, Air Force, and Coast Guard all have very strict rules on this. They have the luxury of being very picky and they use it to their advantage, and rightly so. I believe those services would be the most resistant due to the possibility of a policy like this defeating their current MO.

For example, If you're a CSM in the Army and want to join the Marines, having never served in the Marine Corps before, you would be joining as an E-2 (PFC) and go through Marine Corps boot camp. You would not regain your previous rank. You would go back through the ranks all over again.

My first thought was that we already allow transfers between the branches but of course we don't. We allow some people to leave the service and then reenter another branch. We shouldn't definitely allow transfers when it benefits the services!