Once, civilian ships could be drafted with guns bolted to the decks and Navy crews put on board. They were useful for scouting or patrol work. They provided numbers that the active Navy could not provide. They couldn't fight first class enemies, but they provided needed services.
To create auxiliary cruisers, we could build and stockpile modules in shipping containers that include missiles (SAMs and SSMs) and fire control as well as modules with gun turrets for smaller weapons, 57mm or smaller. Other modules could support helicopters or UAVs. Other modules would contain the command and communications gear to plug the ship into the Navy network. Naval reservists could be assigned to these modules and could train with them during peacetime.
In war, the modules could be attached to the decks of conscripted civilian ships and create instant warships. If plugged into the Navy's network, they'd contribute tremendous offensive power at low cost. In this case, they'd be a tremendous asset compared to the traditional auxiliary cruisers. And with our current fleet of few but high quality ships, the lack of numbers means we simply can't go everywhere we need to or must use a ship with more capability than needed because nothing else is available. Auxiliary cruisers would provide the numbers we need in war.
The hope that the Littoral Combat Ship (LCS) would prove to be a capable but cheap ship that can provide numbers is fading:
The LCS features a number of major innovations. For one thing, it is highly automated, and has a crew of less than fifty. The LCS has a large cargo hold that can be quickly fitted with gear to turn it into a mine clearing ship, a surface warfare support ship, a submarine hunter, or just about anything (anti-aircraft, commando support, or even command and control.) The development of the LCS has been screwed up, with resulting delays and cost overruns. The same grief is expected in the development of the specialized modules.
Those modules are pretty amazing, but this part is what I want to highlight:
Most of the firepower, however, comes in four metal canisters filled with a new U.S. Army missile system called NetFires (or NLOS-LS). This is still in development. This weapon is actually two different missiles, identical in weight and size, but different in how they operate. The LCS is using PAM (Precision Attack Missile). This is a 178mm diameter missile that weighs 120 pounds, and has a range of 40 kilometers. PAM attacks from above, with a 28 pound warhead. This enables it to destroy boats, and damage larger ships. PAMs are vertically-launched from what looks like a 4x6x4 foot (wide x deep x high) cargo container. Actually, it IS a cargo container. The missiles are shipped from the factory in this sealed container. Each one ton container holds 15 missiles and can be carried on the back of a truck, or a ship. Once you plug a PAM container into the wireless battlefield Internet, the missiles are ready to fire.
This fits right in with what I wrote about. The modules that can be built for the LCS could be used for modularized auxiliary cruisers, and by increasing the module buys, perhaps we can get the costs of the critical modules down so that we can afford more LCS than seems apparent now.