Possibly less chance of dust. If the D600 was sent in for a dust/oil problem that would have been fixed and inspected before Nikon certified it as refurbished. It won't guarantee the dust will not return after a few hundred shutter actuations.
The thing about the dust is that it can be easily cleaned. I just purchased a refurbished D600 from Adorama for a great price plus they have a deal to extend the 90-day factory warranty for 1-year with their VIP plan. My D600 only had 19 shutter activations on it. It works fine and no dust at this time.
If it does get dusty, I'll either clean it or get it cleaned. Dust is a fact of life with DSLRs and lenses.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"