There are many many times that I choose not to buy something from a new vendor after going to retailmenot.com and finding no relevant coupons available. There is definitely a value-add there.
I agree, definitely customers who do that, but it is a numbers game. Is enough "new" traffic generated from RetailMeNot to sustain the affiliate links? I am not sure there is any way to know for sure at the scale affiliate programs are typically used.