Only one/two per household (which I am already at with a few manufacturers :p), lost in the mail, bunch of ridiculous requirements like mailing it off within X days of shipment arrival, and don't forget, the value of your money being "tied up" for X number of weeks while they get around to sending it to you. MIR prices are sort of a scam, it has to be an awesome deal for me to even consider it with the MIR. Also, if you value your time at anything, is $10 worth an hour of your life filling out forms, buying stamps, envelopes, etc, and mailing something?
Plus nowadays they usually send a prepaid card. You can look around the web at the problems people have with them. MIR is designed to create an impulse buy as you rationalize the lower cost but they depend on some significant percent of them never being redeemed.
I avoid them as well and it even puts me off the companies that use them.