I've had no problems getting BB to validate a MIR on an item I've used past 10% coupons to purchase.
I think the "cannot be used with other offers" refers to anyone that might have a percent off (or dollar off) coupon they want to use along with this coupon. For instance, when I signed up for my BB credit card, and received it in the mail, it came with a $10 instant gift card coupon (when I charged more than $100). If I tried to use that with a 10% off coupon, they would give me the choice of one or the other, not both.
Basically, for me, with 8.25% sales tax, this pays my sales tax plus a little bit. Nice if I have something I want to buy, but it won't cause me to make a purchase I wasn't already planning on making.