Way back when, a certain UK based CL44 operator used what were called FME's (Flight Maintenance Engineers), qualified both as ground engineers and loadmasters. They certainly proved their worth, offload aircraft, fix any 'stopper' problem & load aircraft. As Nineiron said, the ability to sleep in the cruise was essential as, not being hours limited, they could work all the time the aircraft was on the ground while the crew were getting their 12 hours horizontal. If the route was covered by a slip crew, then there were 2 FME's available, one to load & one to 'fix'.
I'm sure it did look good on the balance sheet but it also got a lot of aircraft back in the air quicker than waiting for a ground engineer to arrive on site!
Don't knock it until you've tried it!
RT