They are one of several resources that schedulers use to list opportunities. You might find something you were not aware of. The company that started the site used to schedule restaurant shops for several clients. They are the first company I shopped for.
It's okay. I got a few leads from being registered. The registration is simple and painless, and because they have a multi-faceted platform of both mystery shopping companies of various types and merchandising, I think it's worth it.
Here's a view from the other side. It's free, so why not? I get an email overnight whenever a new shopper registers in an area where we have a job posted, and I send those to my scheduling team. I know some people are getting shops from them.