Yeah, I agree. Three things are driving the additional C-17's / C-130's (besides domestic politics and support to key districts, of course!):

1. Expansion of the Army and Marines means more airlift in general is needed.

2. The current fleet of aircraft are flying a lot more hours than originally anticipated, so airframes will wear out faster. More airframes mean the fleets will last longer before a (probably expensive) new aircraft is bought.

3. Operational demands. We simply need more to meet the demands of current ops.