[Imp]

Suhiir I have no military background but as US operates 2 forces always thought USMC role was fast response to any situation hence needs flexibility built in. Not expected to go head to head in a full scale war but more to hold till the big boys can turn up then they fill any holes.
As a civvy the odd specialist force for common problems makes sense but on the whole units should be multirole capable of dealing with any threat in an ideal world. To this end cross training & the building block approach makes perfect sense as you pick a force for the expected threat. Believe this is how the Brits to name but one work & I would think the major thing needed to allow this is good logistics support as a flexible force needs flexible supply.