The short answer is that NOAA Corps Officers are cheaper than licensed deck officers in the long run, and therefore better for taxpayers. The longer answer is in my points below.
Some things I saw in the comments I might be able to clear up:
-Deck crew can not hawspipe to NOAA Corps Officers. They would have to go through the same application process/Basic Officer Training Class as the rest of the Corps. There are a very small group (three the last I heard) of USCG licensed mates that hawspiped up and rotate onto ships to train officers (mainly on trawling) and fill in if a Corps Officer needs to take a trip off.
-Occasionally NOAA Ships will bring on crew from the union halls. Mainly when there are not enough NOAA Professional Mariners available to round out the crew of a ship.
-WWII was the last war that NOAA Corps Officers were heavily involved in a war. NOAA Ships occasionally get called to perform diplomatic functions where a warship would not be appropriate. The most recent I am aware of is May 8, 2016 when the Nancy Foster was the first US Government ship to make a port call in Cuba.
-NOAA Corps Officers are assigned two years on a ship, and then three years on a “land” assignment. During the sea assignment, many officers sail 200 days/year. The land assignment is a position in any of the NOAA line offices, such as the National Marine Fisheries Service. A lot of positions involve managing or operating fleets of small boats and divers, designing and working with AUVs and UAVs, or a position that might require strange hours or living conditions, such as maintaining science equipment in Antarctica over the winter. The main purpose of the land assignment is to give the officer a break from sailing while allowing them to work closely with scientists that might end up on a NOAA Ship.
-NOAA Ships do a lot of strange maneuvering that is not typical of a merchant ship. Some examples include holding station for hours without DP while instruments are lowered thousands of meters, deploying/recovering large weather buoys, trawling, towing plankton nets, running survey lines, ect. While I am confident that any USCG Licensed Officer could learn these operations, the “rice bowl” provides a lot of continuity and ensures that officers are trained on these ops early in their career.