They handle consumer complaints. With the proliferation of review sites, I don't see where they would do much for someone in our biz. They're really more of an intermediary between the consumer and a company that won't stand behind their promises or their products. And if you don't join, complaints anyone might level at you don't mean anything.
You know, I could actually see some benefit. But it's expensive so I don't know that I'd do it for a 3-room B&B, for example. 9 rooms, maybe.
If there is a complaint, whether you're a member or not, they'll contact you to get your side of the story and decide whether the complaint is of merit or not.
The thing that they'll tell prospective guests is whether or not you have open complaints, that is whether or not you responded to their query.
So if you get a query, I'd say always respond. Lay your cards out on the table and make your case. They're generally fair.
As for being accredited, if you can afford it, it's a nice feather in the inn's cap. And it does get you some exposure on their site.
Have you heard the "pay to play" issue? According to an investigation, in 2010 an anonymous blogger submitted a listing for the terrorist group Hamas. After paying a $425 membership fee, the phony listing was given an “A-” rating. Likewise, celebrity Chef Wolfgang Puck has claimed that he was informed he needed to pay before his rating could be upgraded to “A.”