I don't know the proposal but one possibility that I can imagine is that a C-17 couldn't deliver loads directly onto the carrier no matter how many times it could fly back and forth. That's not to say that an HAV could do it - I don't know.
I think the argument given for its appeal in terms of cargo is related to being able to collect and drop off point to point. There is the added claim that those points possibly lack the infrastructure that even military transports require.
I presume (because I have no right to claim knowledge) that rough-field landings aren't free of cost for large transport aircraft - there must be maintenance issues. Perhaps the equation is different for an HAV?
A consideration for the future is that the 20t and 50t HAVs are minnows compared to the 1000t version but a start has to be made somewhere.