I don't know if this is a fact, but it's my personal opinion on rebates ...
I noticed that rebates really started to increase with the past few years. I also noticed that alot of places changed thier PM policies (ie staples used to match %155 percent, now they only PM). I think that the rebate thing happened so that people would stop doing the price matching and so that the people would go to the store and actually get it (as well with the other reasons people suggested).
But like, if you see CD-R's at best buy for $3 after the MIR, but like $23 before it, you can't go to staples and do a PM on these. And for me, that is a 50 minute round trip as opposed to a 10 minute round trip that I used to make before these MIR's.