They are indeed very good and lets say compared to a KM Engy. Once over vetted. You are just losing on some OH but gaining on repair stats unless you are using Heros to compare.
That being said. They really shine for a HA/HD. I can field 6 Nations of BB6-BB7 using the Hired sailors I have. MN is special as they can use both at the same time. So you need 1 Scout. 2 Gunners. For me, 2 Regular Engys. 5 Guys(6 if using HH) per nation. The rest being Hired guys. I can run 6 Nations. So for me consider that 12 BB6 and 3BB7 and a BB6.5. You never run out of Them in 2 hours. You never have to worry about gettinng sailors out of HQ as long as you set up the day before.
So people dont think I cant add. I have 6 Hired Axis Engys. 7 Hired Allied Engys. 1 Axis Rep. 1 Allied Rep. 1 Axis BO. 1 Allied BO. This equals 17 sailors.
5 sailors x 6 nati.ons = 30 sailors
30 + 17 is 47 . Which means I have room for HH gunners if I choose. That depends since the BB7s require 1 extra support sailor.
They are also helpful for leveling. You dont have to beat up your real crew and they save room in your Harbor so you can have full blitz crews ready to go and a few other crews ready to go without going into HQ.