I ran support for a mid-sized integrator for a few years. We resold manufacturers products and made none of the stuff ourselves (except we built OEMed Geovision PC-based DVRs) and I can tell you that the entire RMA process is a big friggin game.
As the reseller, whenever something failed at a customer site, the integrator had to troubleshoot the device on-site (with us, on the phone) before we issued the RMA#. This process annoyed the crap out of the good techs since they had already done everything to isolate and eliminate components to determine the fail point...i.e. bad device, but it saved us thousands (perfectly good, now 'used' (B) product on our shelves; shipping/handling, etc) because lots of times the integrator just wanted to return the stuff because __________ (insert whatever you can imagine).
If the integrator can't use the product they ordered (for any reason at all), they are not going to eat it if they can make the supplier eat it. If we received a vetted RMA, and it worked (with 24hr burn test), then they got the same device right back. :) ...but I told them this would happen before they sent it to us.
Then, if I get the device back and it doesn't work, I have to play the very same game with the manufacturers support person before they will issue me an RMA# - which annoyed the crap out of me, as I had already tested it twice. If they got the device and it worked (don't know if they burn tested), they would send it back to me - and it would become B stock, magically losing around 40% of it's value because the buyer didnt want it/couldn't use it.
As far as used vs new as a replacement for an RMAed device, I agree with Carlton - unless the device was doa (or failed within a very short period of time) I wouldn't think a new device is a legitimate expectation. That said, as we did back in the day and I still agree now, you must disclose the policy and everyone needs to be aware of what it is.