There is one site I have used in the past called Lookfantastic, and they always offer discount codes, as I get an email nearly every day, but the exclusions are on over 300 brands, so why do they offer discounts when they are so limited? Yes, it maybe to get people to go to the site and when the code is invalid they hope they will still checkout anyhow. The opposite has happened where more people complain about this, and end up writing bad reviews. I think if you offer a discount, yes, there will be exclusions, but they should be limited and not in the majority.
I'm highly distrustful of these coupon sites. Note, I used to go to them to try to get discounts on domain renewals. However, they seem to always be lying. Anyway, I wonder there are enough good codes on the site to make it worthwhile to visit. Note, these sites probably get good traffic so there must be some.