It makes sense since apparently chronically understrength platoons often formed squads into two fireteams anyway in Iraq / Afghanistan, and this is why I believe a reserve element should be built into all echelons of the TO&E (reserve fireteam at platoon, reserve squad at company, and so on). With a primary function of serving as a buffer for immediate replacements to keep end units as full strength as possible for as long as possible (for losses either due to admin / training or combat) there is otherwise no shortage of tasks they can be assigned, from HQ element security to guarding caches, or carrying out resupply or other minion work that would otherwise require warm bodies from the line.  

While I disagree with the USMC equipment choices I do like 6 man fireteams, and I would emulate a similar structure for mechanized forces but add a third fireteam for light infantry (also expanding the weapons squad). Span of control for squad leaders remains the same as in the traditional 13 man squad with three firearms (1:3)