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.