In the case, of DELL they want to make sure the machines that are returned do not come back again to them again. I think there is better testing on these refurbished machines than the new ones.
Of course, some returns are not because of failures but of configuration errors, such as the $24k server I mentioned. That one had a 32-bit OS installed and required a 64-bit one (or it may have been the Workstation laptop it got). Though these configuration errors seem to be not the majority issue on the DELL site.