To be honest, if I had my choice of having to defend a support unit of marines or a support unit of Army personnel, I'd take the marines any day of the week. At least they know how to shoot, move and communicate.
One problem that Hack didn't discuss is the problem of support units not taking their infantry duties seriously. I've heard some real horror stories and have a few of my own about support folk, or REMF's as we lovingly call them in the Army (rear echelon mother f-----s).
This is what will bite us in the fourth point of contact when we fight another WW, which we will, regardless of what folks like Thomas Friedman and his ilk believe (i.e., history has ended and wars are obsolete).