I can't see what value a USN base in the NT or the NW would have.
As a liberty port, it is no Singapore. As a maintenance and support port, ditto. It isn't on the way to or from from a ME deployment from the West Coast or Pacific bases.
In a time of future regional tensions, if the USN is squeamish about sending a carrier group through Singapore and Malacca, then they aren't going to want to risk any of the Indonesian passages and the Torres Strait is too shallow, even if they could get into Darwin. Which means going around Australia anyway.
As a location for pre-positioning equipment, it is no better than Diego Garcia or Guam but is significantly less secure from a USN perspective. And given the kinds of tidal movements and seasonal weather you get at Darwin, you may not wish to use it as a long term anchorage when other options are available.
For pre-positioning ground equipment for exercises, I don't think the NT training areas are capable of the kinds of sustained military activity to justify it. If you read environmental reports they are quite fragile environments that require careful management and rehabilitation after intense training activities. It is also true for Queensland training areas, especially SWB.
Crew swapping would be best done in Singapore or (as has been mooted) at HMAS Stirling (especially SSNs).
On the other hand, as a logistics area for pre-positioning of bulk provisions or relief supplies and other aid then it is excellent. NT has lots of space, which is quite expensive in Singapore, and is more secure than elsewhere in SE Asia (physical and political).
The NT would be a good place for the support of air assets and positioning of certain supplies and equipment, such as ordnance for use during exercises or on Delamere, but Tindal would probably be a far better and more secure location, as well as being less public and probably less controversial.
I think all this talk is more political than actual need. There is a desire to engage the interest of the US in the South-West Pacific region, and this might be part of it. Personally, I think the US value their relationship with Australia for things other than real estate on which to base weapons systems (such as real estate for sensors and aerials
or even those long rumoured UFO hangers under Pine Gap
nfloorl: ).