@Hair Warrior hit the mark pretty close. I am not talking about converting a tanker into a warship, I am talking about having the components in place to quickly make the conversion in time of war. Peacetime...ship oil. Wartime...I recognize the concept is more complex than I am making it sound, but imagine taking ten tankers (average 1200 feet of available deck space) and fitting them with a several HIMARS type launch systems - basically "the gun in the drawer" you drop into the holster (the ship) when you need it. Using the standard HIMARS launch box as a measure, you could drop in about 300 per tanker. That equals 900 rockets each with a range of 190 miles. Add to that the fact that there is likely better rocket/missile technology with greater range and payload and you have a pretty deadly system out there. Worried about enemy subs...OK, let them go hunting along the merchant shipping lines, it will take pressure off the primary fleet.
I am not in any way advocating that we cease building warships. I think we should build as many as we can afford to properly and effectively man. Nor do I consider the idea binary...we can do more than one thing at varying cost. Nevertheless, if you take the US fleet in the Pacific (say 95 warships, not auxiliary) and add in the entire 96 warship fleet of the Japanese, the 50 warships of the Australians, and South Korea's 120 warships and the Chinese still have a slight advantage in overall strength. The ability to quickly add some firepower to the mix at a small cost can't be a bad thing.