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'd rather an instant rebate that I don't need to damage my boxes with or that relies on my memory, etc.
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.
You can damn well bet your life they were trying to wriggle out of it. If you no longer had the receipt, your rebate would have vanished down the plug hole.
Had the rebate have been "instant", you wouldn't have had to rely on having the receipt. You also would have had the extra $450 from the moment you purchased the equipment, not some months later.
If you went to the bank and asked them to lend you $450 for a couple of months, interest free, do you think the bank would say "yes"? Hint: if you withdraw cash from your VISA/Mastercard and don't pay it off, it'll earn interest at around 20%. But you, in your infinite wisdom, gave whatever company it was you bought this off $450 free of charge for 2 months.
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.
Right - people get attracted by the "$400 rebate" but then forget to post it, post the wrong details, gets lost in the mail, etc, and the customer never gets the discount that they thought they were going to get. And this ensures that the scammer never has to give out the rebate to every person.