Mail in rebates are effectively you loaning the company you are buying the equipment from the rebate money.
So all of you buying a 6D or 5DIII and looking forward to that $400 extra, well what you've done is said "Here, B&H, you can have $400 of mine for free!"
Mail-in rebates typically have a specific window of time during which you must remember to mail in the barcodes (or whatever) or you don't get your rebate. Then it can take a year or more to receive the rebate.
From the time you pull the trigger until the time you get the rebate, you have effectively loaned B&H $400 for free.
Mail in rebates are a scam and should be avoided like the plague.
They're only a scam if you don't get the rebate, and only a nuisance if you mind having to remove the barcode from the box. If you would rather pay full price, go ahead.
I recently got $450 worth of rebates on three Olympus lenses I bought along with my OM-D back in March. I initially thought they were trying to wriggle out of it when they told me that I had sent the wrong barcode for the camera, but they ended up just accepting my receipt and sent me the full rebate anyway. It took a couple of months overall, but I paid $450 less than I otherwise would have done.
Plus, if my experience selling Tamron lenses I bought while I owned a Pentax body is any experience, losing the barcode via a mail-in rebate doesn't affect the resale value of the lens to any significant degree.
I suspect that the only way companies get out of paying such rebates is if the customer, having been lured by the rebate, forgets to mail it in in time or doesn't want to cut out the bar code; presumably enough of them do this for companies to keep doing it.
