If you use enough of the same equipment next thing you know someone will be asking "why do we need the Marine Corps"?
At what cost though? If it starts to cost too much then people will question its existence then. Its a balance, and in this minor case I don't think it is worth it.
P.S. Hard to to see how using the Blackhawk would be that much of a detriment either, as far as I know it is the only aircraft used by all 5 services.