I've had plenty work. Maybe you are thinking of bugmenot?

Some venders embrace retailmenot and thrive, some make them pull all codes and probably suffer for it.

I've tried one from retailmenot in the past week or so which failed, but I think you're right, it's bugmenot that rarely works.

I've had a ton of them work, especially for NameCheap and GoDaddy.

I'm sure I've used a code from RetailMeNot more than one hundred times. It is a useful site and the success percentages are reliable.

I've found it works about half the time. Still good enough for me to bother checking. They have a bookmarklet.

Yep, I had one work.