Marine Band vs. Field Bands
How in the hell we got on this topic I will never know, but FYI:
Members of the United States Marine Band, "The President's Own," are NOT Marines (except for the Drum Major(s), any prior-service Marine members (recruited from the field bands), and the logistics crew assigned to the Band). They are enlisted at the E-6 rate solely for service with the Marine Band, have an internal promotion system, but are still subject to the UCMJ. If you look at their rank, you will notice that in place of crossed rifles, they have musical lyres.
Why, you ask? Marine Band members are some of the finest musicians in the world. For the most part, if they do not play their instruments for even one day, their performance suffers. Since they enlist for assignment to the Band, and are non-deployable (they cannot even leave the D.C. area without Presidential approval), it is unrealistic to send them to boot camp, if they are simply going to serve as musicians.
Marine Bandsmen from the 12 field bands (in the 5500 Occ Field), and the Commandant's Own Drum and Bugle Corps, on the other hand, are every bit Marine. They go through boot camp, MCT, and are held to the same promotion/PME/UCMJ/etc. standards as every other Marine.
Why, you ask? Marine field bands "belong" to the CG of the Division/Wing/Base/Depot/etc. they are assigned to. While their primary role during peacetime is musical support, their secondary mission is headquarters security. (The III MAW and I MARDIV bands were heavily tasked in the past year - in fact, I MARDIV band is deployed as we speak.)
It is worth noting that Marine musicians must be MOS proficient PRIOR to enlistment - it is one of the rare fields where MOS proficiency is a prerequisite to service.