The individual battalions do not remain with thie regimental HQ. There are several reasons. A. The Marines have long practiced task organization. That is companys and battalions appropriate to a specifc mission are grouped together. Assigning missions to rigidly organized units is grossly ineffcient. This was well proved in WWI and the US military was well along the way to a more flexible system in WWII. B. Politics, budget dictates, capacity of maintiance and training facilities, mean that a rigid organization cant be accomidated in one location. It is necessary to fit battalions in where there is room. Not according to a antique organization chart. C. Individual vs unit rotation. For many years the Marines rotated individuals in & out of the rifle battalions of the 3rd Marine Division on Okinawa, on one year tours. This made those battalions less cohesive and less a combat team. It also caused problems with the battalions in Hawaii or the US when men were drawn out to go to Okinawa. By swaping entire battalions from the other regiments to Okinawa the problems of individual rotation could be reduced. By shuffling rifle battalions about, and some companys such as from the tank battalions a more flexible and effcinet organization can be had.